Another suggestion is to set up something that e-mails past attendees with a quick poll of whether they are coming to the next meetup (1 extra per week is likely worth it), and there is an updating thingy in the LW post that shows accepted/tenative/declined vs total number on the list and time to next meetup.

I don't know which parts of this would be difficult to implement, but it (working with the final product, not necessarily setting it up) is easier than having people answer an LW poll given the complications posted in other comments below.