Disney lists the dinner as "a la carte". So you should be able to order dessert only, or anything else you would like.
The menu doesn't list individual prices yet, so it may be that information is incorrect. Or maybe they will do
both a la carte and prix fixe-
"...after all Miss, this is France!" & that's kind of a French thing to do!
In addition, I think the menu is pretty extensive for a restaurant that will accept the
DDP at 1 TS credit. That is, there are a lot of things on there that won't fall under the DDP. It's usually packages that are prix fixe, (e.g. Fantasmic! or the Candlelight Processional) or really high end restaurants like Victoria & Albert's.
It's a good question, and one I'm definitely going to check back on when we go in December, because my family typically grazes all day, and if it's prix fixe, I'll have to starve 'em a little bit or they won't eat it all!