A game jam would definitely be nice, but beware that the LD is soon, and I don't think too many of us would enjoy having two jams in a one week period. :p
Maybe organizing regular jams could be a nice way to show the SFML community is alive and running.
I think that a 72 hour jam would be nice, but you could host two, one of 72 hours and another that lasts a week, and do the first on the first week, then the second on the next week. Dunno.
Also, it's nice to have a game jam that only includes a certain library. I hate the competitive bit of the LD because of all the people using GameMaker and Unity and whatnot, when you're working your ass off in C++.
It would definitely be something I'd participate in and enjoy. I guess we'll see how the idea turns out.