Yep, if your game is incomplete and you still want to submit it - that is definitely OK :)
Cool! Luminy, can't wait to see your project or demo. Anyway it's quite difficult to release a complete Myst game before the 18th (by 'complete' I mean a game with several hours of gameplay - just like Myst). I wonder if some participants are working on it by teams and since several weeks already. When was the Jam announced originally? We only discovered it late on a French magazine.
I announced the jam in early December I think. Anyway the jam is specifically for making myst games, rather "games inspired by myst" (but if someone wants to make something especially myst-like that's super cool too of course!) so yeah I wouldn't expect anything several hours in length unless someone is doing something procedural.
Thanks Icehouse for your encourage, also I can't wait to see your entry ;-)