Per the whitepaper linked below, OpenCL support exists only in Bullet 3.x, not Bullet 2.x:
bullet3/GPU_rigidbody_using_OpenCL.pdf at master · bulletphysics/bullet3 · GitHub
I haven’t scoped out the transition from Bullet 2.88 (what Minie uses) to 3.x in detail, but it appears nontrivial. Also, I haven’t found evidence of any Bullet 3.x releases; I suspect the code is still experimental and not ready for production.
I’ve decided not to pursue OpenCL right now.