Well, having it written into the 504 plan has helped. Because you may remember: the first day of school in fourth grade at a new school (new town, new state), DS refused to sit at the table until someone came and wiped it to make it safe for him. This was pre-504, but we did have a plan in place the school agreed to.
I got called into the principal's office that day. Seriously. Because my DS wasn't being polite. According to our very southern principal, it was a huge offense.
And at that same school DS got in trouble for not cleaning up another kid's lunch trash that the custodian told him to.
So he apparently doesn't have many problems with this
BUT he was really struggling with when to be polite v. when to speak up, and putting a clause in DS' 504 that his efforts for self-advocacy will be affirmed/respected helped him understand that he can speak up without fear of recourse.
That's not exactly what you were looking for, but I felt I needed to include it. There are outside resources, and there are examples of giving a child tacit permission--and even expectation--to speak up in a document that all of his teachers read every year.