Brian, there was some discussion earlier this year regarding the use of soaking curved antler pieces in a solution of vinegar for some time (at least a couple of weeks?) then taking them out and putting in a vise for a while. I believe Tim C tried this and worked pretty well. I currently have a couple of pieces soaking now and will be a bit longer before putting in a clamp.
Both Tim and I, as others, use antler pieces for tuning as tips for powder horns. I don't see why they wouldn't work for knive handles either.
And, sometimes after requesting a 'straight' piece of antler, I get a 'slightly' straight piece from a vendor. So far I've been having real good luck dealing directly with an Ebay seller that actually delivers what you request. Now if this vinegar solution tends to work then I won't be too upset after getting only 'slightly straight' anymore.
Gary