My advisor suggested getting a letter of recommendation from a former research assistant as part of my job materials for teaching focused schools. This would not be one of my required academic references from faculty, but would be submitted as supplemental material. I've read elsewhere that applications can be thrown out for including extra material and "not following directions." So I just wanted to get thought on this - is a supplemental letter from a research assistant a good way to demonstrate effective mentoring or will it get my application dismissed?
As a SC member I would not throw out your application. I would consider that to be part of your teaching portfolio. I'm not sure that it is really necessary, but I guess it couldn't hurt.
As an applicant, I included letters from students/RAs as part of my "evidence of teaching effectiveness" package, or my teaching portfolio. I am applying mostly to teaching-oriented schools, and they all asked for one of those 2 documents, so I wasn't sending them anything that they didn't ask for.
make sure you include it under "evidence of teaching effectiveness" rather than as a letter that is supposed to be on par with the letters from your mentors.
A friend and former labmate who recently got a job at a very elite SLAC included quotes from a mentee letter in her teaching statement. They were integrated into the section about mentoring (i.e., she talked about her goals and style as a mentor and then provided quotes from her students as evidence of her effectiveness). Not sure if she separately sent the letter in its entirety, but obviously this approach was effective!