You know when you're in the dentist's chair and the dentist starts to drill out one of your molars and you swear you can smell your teeth burning? That's what your shop's going to smell like when you sand those antlers down! From experience, DO NOT let the wife in your shop for days, or she won't let you use antler again!
Yeah, the belt sander is the best bet... Antler is also VERY porous, so I would recommend one of those epoxy squirt tubes that you can get at Lowe's to glue the thing up. I think Loctite makes a good one for around 6 bucks. Just rough up the wood a bit and lightly clamp - no heat, no mess, no fuss.