How about this: Before a mixer, tell dancers that if they've booked ahead
you will give them special dispensation to put off that partner for this
one dance, since it will be a mixer. So go find someone you've never danced
with (or not danced with tonight, or some schtick) because this is, after
all, a MIXER dance.
It was dealing with this issue that got me married. The first time Christine
and I were partners for a dance slot, at the Glen Echo Friday Night Dance,
it turned out to be a mixer (with no prior announcement). So we booked the
next slot to do a contra together, even though I didn't regularly book
ahead. All of that gave us enough time to start talking . . . and now here
comes our 4th wedding anniversary.