Personally I think you can teach or your can't, so not all volunteers can successfully be put in that position, especially socially awkward ones.
As a generally socially awkward volunteer, I put in many many hours teaching basic mechanics. I don't think I'd be much good as a teacher in many other contexts, but with small groups of keen learners people often left smiling and even more keen to wrench.
I found it useful to have many ways to describe things, and also had a lot of success having people try stuff even before I'd do much demo, then doing demo to show a little trick to make it faster/easier or to get better leverage etc, then having them finish the work. I don't have much patience otherwise, but it came pretty easy once I got an idea of just how many ways people can learn the same task.
Anyway, I just think teaching bike mechanics is pretty well suited even for people that might not be much good at it otherwise, your misfits might surprise you.
Jeff