In the corner
SA OC 21, 1:
חוטי ציצית שנפסקו יכול לזרקן לאשפה מפני שהיא מצוה שאין בגופה קדושה אבל כל זמן שהם קבועים בטלית אסור להשתמש בהם כגון לקשור בהם שום דבר וכיוצא בזה משום בזוי מצוה.
... as long as they are tied to the Talit, it's prohibited to utilize them. E. g. it's prohibited to tie with them any object because of the disrespect of the mitsva.
have some extra string and I was curious if it would be considered disrespectful or impermissible for someone to use a Tzitzit as a keychain
--> prohibited because disrespectful
You can see a beautiful pilpul in the Taz sk 1, with a svara to allow but halacha lemaase it is prohibited.
Out of the corner,
Lich'ora there is no mitsva, so no disrespectful use.
One can ask: perhaps the strings were made with intention to be tsitsit of mitsva only by neder (e. g. if the owner said the statement "leshem mitsvat tsitsit") ,
but a priori it's not the case, despite the fact that one said 'leshem mits at tsitsit'; the 'leshem' is regarded as a beginning of mitsva not as a neder, and generally the workers who say it are not owners and cannot prohibit the wires. The end of the mitsva is not mandatory; when the mitsva is not continued, the first steps are not sufficient. BTW, there is a mitsva to make Tsitsit, as there is a mitsva to make succa, which both need beracha of "asher kiddeshanu" according to the Yerushalmi (for the halacha wed do not follow the Yerushalmi). But the tsitsit still not became a chfets shel mitsva.
When a mitsva was already made, two opinions exist (about a wire which was previously tied for tsitsit of mitsva) concerning disrespectful use,
but when it was never used for a mitsva both seem to allow any usage.