Mistakes in Berachot are discussed in Masechet Brachot in two places. We will examine one of them, the relevant one for this question.
When someone - 1 - already said the name of G-d, and when - 2 - only one Beracha is needed, (not a couple of consecutive Berachot as in Havdala), and - 3 - the said Bracha and the needed Bracha are linked (in their function), e.g. two Berachot which are said before food eating, the Gemara Brachot 12a {1} can give some answers. You wrote:
I accidentally say asher yatzar when I'm commencing a berakha over food.
Tosfot {2} reports diverse opinions on this Sugia, Rif principle of Kula, RI principle of Chumra and RaCh. Following the principle of the Rif, there is a Safek Derabanan, leKula. The psak, is issued from the non-conclusion of the Gemara. It is allowed to correct the blessing statement Toch Kede Dibur. And the Shem Hashem and Malchut was said in a approximately right intention, even if destined to an other kind of Bracha.
The Chidush of Rach (Note that this pshat on Rabenu Chanan'el is based on the reading of the Tosfot, not on Rabenu Chanan'el on the daf. It is also different from the pshat of Raavad in sefer Catuv Sham and on Rambam hilchot Brachot chapter 8).
Tosfot described following RaCh a special situation, similar to this described by the OP. Here are the two examples described in Tosfot. First case: A man knows that he holds a glass of wine and inadvertently, he blessed something else; second case: he knows that today is Yom Tov and the prayer is the prayer of Yom Tov, but has inadvertently said "Mekadesh Hashabbat". According to RaCh, these two examples are equivalent to the cases of the Gemara, because here, there is no mistake concerning what is blessed (wine, Yom Tov) and how to bless (Knows the right Bracha), only there was a discrepancy between the intended and the spoked thinking. So, the Shem Hashem (and Malchut) was well oriented (right object, right intentional thinking).
According to this chidush of RaCh, if you wanted to say Asher Yatsar and kown why, but your mouth said Hamotsi Leshem. You can rectify toch Kede Dibbur.
Without the Chidush of RaCh, it is less obvious, may be not good and you need to repeat the whole Bracha (to answer this side see Raavad). RI also explained that the problem of the Gemara is that a Kavana for somewhat else is different from lack of Kavana, it is an opponed Kavana for an object excluded from the Bracha is an opponed Kavana to the Bracha, not only a lack of Kavana. (this is an important svara for understanding several problems in this suggia). See SA OC 209 1-2 after the reading of Tosfot. The Shulchan Aruch follows the Rambam on his first words, and in "Yesh Omrim" follows the Pshat described above (Rashi and Baal Hamaor). Magen Avraham stated as the last. Mishna Berura too (209 sk 2)
So:
If somebody does start saying asher yatzar in a berakha that does not require it, should s/he stop and start again from the beginning?
--> No, toch Kede Dibbur he need to correct and properly blessing after Haolam.
Is there a fall-back berakha?
--> after Hashem, "lammedeni Chukkecha" (Tehilim 119), ruled in SA OC (206, 6). E.G if the fruit falled after he finished the word Hashem. But it is not a Bracha, it is a verse recitation.
Would quickly turning this into a berakha for something else be problematic - ie: because the individual making the berakha didn't have that action originally in mind?
--> Yes. It is ruled by the RaCh in Tosfot and stated by the Shulchan Aruch in the sayif 2 of the above cited siman.
Sources:
{1} Gemara 12a.
פשיטא היכא דקא נקיט כסא דחמרא בידיה וקסבר דשכרא הוא ופתח ומברך אדעתא דשכרא וסיים בדחמרא יצא דאי נמי אם אמר שהכל נהיה בדברו יצא דהא תנן על כולם אם אמר שהכל נהיה בדברו יצא אלא היכא דקא נקיט כסא דשכרא בידיה וקסבר דחמרא הוא פתח ובריך אדעתא דחמרא וסיים בדשכרא מאי בתר עיקר ברכה אזלינן או בתר חתימה אזלינן .
There is no question that where a man took up a cup of wine thinking that it was beer and commenced [with the intention to say the benediction] for beer but finished with that of wine, he has fulfilled his obligation. For even had he said the benediction, 'By whose word all things exist', he would have fulfilled his duty, as we have learnt: 'In the case of all of them, if he says, "By whose word all things exist", he has performed his obligation'. But where he took up a cup of beer thinking it was wine and began [with the intention to say the benediction] for wine and finished with the benediction for beer, the question arises, do we judge his benediction according to its beginning or according to its ending? ...
Come and hear from the concluding clause: 'The principle is that the final form is decisive'. What further case is included by the words 'the principle is'? Is it not the one we have mentioned? - No; it is to include bread and dates. How are we to understand this? Shall I say that he ate bread thinking that he was eating dates, and commenced [with the intention of saying the benediction] for dates and finished [with the blessing for] bread? This is just the same thing! - No, this is required [for the case where] he ate dates thinking that he was eating bread, and he began with [the intention to say the blessing] for bread and finished with that of dates. In this case he has fulfilled his obligation; for even if he had concluded with the blessing for bread, he would also have fulfilled it. What is the reason? - Because dates also give sustenance.
{2}
Bellow the words of our Tosfot:
לא לאתויי נהמא. ופירש רב אלפס השתא דלא אפשיטא בעיין אזלינן לקולא ואפילו פתח <בחמרא וסיים בשכרא יצא. ור''י הי' אומר לחומרא דצריך לברך פעם אחרת. ומיהו היה אומר ר''ח אם היה יודע בבירור שטעה בדבורו שאמר בורא פרי העץ תחת (בורא) פרי הגפן דבתוך כדי דבור יכול לחזור בו. וכן בי''ט בחתימה של יום טוב אם טעה בין מקדש ישראל והזמנים ואמר מקדש השבת וחזר בתוך כדי דבור יצא אחרי שהוא יודע שהוא י''ט. והקשה הר''ר יעקב מקינון מאי קא מבעיא ליה והא ודאי מצות אינן צריכות כוונה. והיה אומר הר''י דהיינו בשומע תפלה אחורי בית הכנסת ולא נתכוין לצאת. אבל היכא דנתכוין לברך על היין ונמצא שכר לא מהני:
Note: The Klalim of Safek Berachot Lehakel is true only because the duty of Brachot is Miderabanan. So the example of date and bread in Gemara, which is concerning Bracha Acharona Me'en 3 or BHM, which are Min Hatora for a great enough quantity, is not relevant for the Klal used by Rif for the psak here. (from a logical point of view in Gemara it is not relevant because the Gemara argued this at a searching step, hopping to find the radical rule.)
Nafka Mina for Birkat Hatora and Birkat Hamazon, the rule of the Rif has no utilization and we need to repeat entirely the blessing from the beginning.
