you all have a point, and megan, your right but the band has to know who you are first also, I see nothing wrong with going up there to him introducing myself and saying hey i play the accordian and then going sit down and see what happens. Never asking though.
on a few occasions I've gone to see well known artists ( Flaco Jimenez, Dave Alvin, and others ), with whom I would never dream of playing with publically,but, after the gig, I got to know them back stage, and often an improvised jam session would ensue.. a spontaneous gesture on their part is worth the wait. Otherwise, a nice gesture would be not to do any playing but to get one of their CD's signed, an accordion signed, or leave them with a CD of yours ( even if homespun ).
Good luck ! Joel is one of my Cajun heroes for sure.