You're right; I wish I hadn't phrased it that way. I said this because the last time anyone replied to anything I asked there was early 2012. Just because no one seems to have answers for me there nowadays doesn't mean it's dead to everyone.
There were 4 posts in the support forum on 14 May - not sure how that counts as totally dead.
1. Bullet Physics forums appears totally dead.
Using 2.81 version. It seems to be fast enough for my purposes (about a hundred objects falling on it at once as a demonstration/proof-of-concept). I'll keep that in mind though. Maybe commit your extra test upstream?
By the way, collision of convex shapes against the heightfield is extremely slow (though I'm using 2.79), because whilst it does an AABB check of the convex against the heightfield, if that passes (which it generally will if your shape is near the surface) it goes on to do the full test against all the heightmap triangles in that block - even if they're way underneath your shape. or something like that. I put in an extra test to speed things up.