I was under the impression that it was proper "etiquette" to wait to be asked no matter what caliber of musician you are. My musician friends tell me that they would never think of making such a request. The honor is in being invited up. If the band knows you and respects the way you play - they will invite you to sit in (if it is appropriate to the venue.) Just my two cents.
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.