Good post Julie!
My idea exactly! Go back to the same restaurant (but call first to request the specific waitresses section). That way, you're wife will flirt and the waitress will also know that you guys choose her.
As for bringing the situation up...it is obvious that if you ask her directly "do you want other chicks?" ... her resistance comes up fast.
Maybe say " what's your favorite part of being with another woman " ... that way you're not asking her to commit to some action.
Weave it in in-directly...perhaps make her think it's her idea
if you can.