I sent my invites out in September too, but made the deadline for RSVPs February 6th at the suggestion of my TA - I have received a TON of 'yes's' back, but only a few have booked, so dealing with the same issue. I TOO am surprised more ppl haven't booked, but several just started booking this week (I guess just waiting after Christmas, New Year's, or perhaps til the last minute of my RSVP date?)...
I was going to send out a gentle email reminder, but an Evite would work just as great (and I like your wording). Soooo frustrating when you're just trying to get a head count, I know....good luck though!!