The WVA festival has the whole "four days" - (from Aug 18 thru Sept 19 this year - see the thread) so there's lots of time for "on-site" mixing. With 20,000 people available, a third of whom may play passably, the couple dozen who may get into a given mixup seem to self-select to a fairly even level of skill. I don't know whether there's more variation in the smaller camp sessions, as I've only heard a few.

The two "unofficial stages" that occasionally have done the "blind date" format mostly do the "open mike" thing where you sign up your group to perform and they give you a time. Competition to get a group on is sufficient that you usually need to get in line at least a couple of days in advance.

The mixers may draw quite a few more volunteers than they have time to put on, so the draw is often to see whether you get into a group as well as to assign you to one.

John