I have no idea bout treats, but I think they're always grateful for pinch hits. I qualify for less fandoms this year than I used to myself, because I haven't written enough, or anything, in the new fandoms they added and wrote too much crossover stuff in the old ones, but I still could offer five, thanks to my old Jossverse, DS9 and Highlander fic. (Merlin is the only still open canon fandom I'm qualified in, that and comics because the perimeters there are so boad.)

However, I've found that sometimes you're assigned a remixee who happens to share other fandoms not on the list as well with you, and it's always possible to take those. (That's what I did the second time I did the ficathon, because I got the same writer I had remixed the last time, and it resulted in my one and only Rome story.)