I can't remember if I already answered this or not:
Brioche dough may be too soft for quiche. I would think the filling
would fall right through the dough after baking (unless you used a
really thick piece, but I don't think that would be appealing). Puff
pastry could work. It would be a bit crunchier than croissant, but I
wouldn't necessarily mind that. Heck, I would even use something like
a cream cheese pie dough (Rose has a wonderful recipe in The Pie and
Pastry Bible). Even phyllo dough would be nice (flaky and butter layers--yum!).
So yeah, you could use something other than croissant. Just keep in
mind it has to be a rather sturdy dough to hold the filling in.
Roxanne