Hello , it seems a shame that no one has replied to you, so here goes, for what its worth.
I'm a bit puzzled how your keel can lay flat on one face, but not on the other ? (I assume you mean that it lies flat if you put a 3mm chock under each end , given that it is joined in the middle by a lapping piece of (if memory serves) 3mm (but then this is assuming that you have joined the fore and after pieces of the keel, which may be invalid assumption)).
However, even if it does have a bow/warp, you could do things like pressing it flat and gluing some doubling pieces along its length, and leaving it to dry under weight / pressure. Make sure that the surface it is lying on is definitely flat. By the way i don't mean to patronise you here, so apologies if you have already done this. But probably even if you left the warp in, it would straighten up OK when you glue the bulkheads and stringers in place, as long as you have it pinned down over the plan to make sure that the bulkheads are aligned while it is drying. Probably even then if the keel was a bit squiffy, when the hull skin is put on , you can sand the liteply so as to get the keel line as exposed to the water, straight. I must get on with mine, i have a hull and decks but no superstructure.
Cheers.