How to ask a professor to join a project in a way that leaves him a "graceful out"?
I do not want to end up trapping either of us in an advising relationship that would be bad for both of us in the event he was just trying to be nice when he agreed to be co-advisor on my master's thesis.


Give him some credit and don't assume he would agree to a bad advising relationship just to be "nice."

Ask him directly. If he wants to say "no," there are any number of ways in which he can say it.