The OP is under a mistaken assumption IMHO.
In the linked answer (provided by the OP) to another question on "MY", it was stated (in the name of the Rashb"a; Teshuvos Vol. I: 18) that there is no brachah made on the mitzvah of kibbud av v'em (honoring parents) or Tzedakah (charity) because the receivers might be "mochel" (forego) the obligations owed to them.
I took the liberty to correct that answer by editing the word "mochel" (forego) and replacing it with the idea that fits the Rashb"a IMHO:
The Rashb"a says that there is no brachah made on giving Tzedaka because the poor man might "REFUSE" the gift offered. (NOT MOCHEL)
So too in the Rashba's example of kibbud av, he explains that the case is where the mitzvah is uprooted entirely, similar to when a woman REFUSES a marriage to a rapist or seducer, thereby uprooting the man's mitzvah to marry her.
Obviously the following example of the OP is not correct:
"The father enters the house and the son stands up. The father says "Oh, please, you don't have to, I'm Mochel wholeheartedly". The son says: "Nevertheless, I'd like to show my respect by standing before you".
Rather, the example that fits the Rashb"a is:
The father enters the house and the son stands up. The father says "Oh, please, I don't want you to stand up for me, I'm asking you not to do it".
If the son says: "Nevertheless, I want to do it anyway" it is not a mitzvah anymore because:
1) The father specifically asked him not to do it.
2) Hashem is not overriding the father's request (which would make it into a mitzvah anyway), because we know that a father who is mochel his honor, then his honor is mochel.
Similarly if you walk by a poor man and he outright "REFUSES" to take charity because of his pride, you will have made a blessing but cannot perform the mitzvah.
Therefore, we don't make such blessings.
Now if you will argue that the son knows his father doesn't mind if he stands up for him...then he should make the brachah?
The Rashb"a explains that once a situation like that, (uprooting the mitzvah based on someone else's wishes) in a mitzvah, is merely possible, then Halachah never established a brachah for that mitzvah, regardless of your knowledge about the person's wishes in a particular situation.