Can't they use a software renderer like swiftshader? You don't need to pass in an actual gpu through virtio or whatever.
Maybe you can call a WebGL extension that isn't supported. Or better yet have a couple of overdraws of quads. Their bot will handle it, but it will throttle their CPU like gangbusters.
Sounds like a PoW system with extra steps?
It's exactly a PoW system, but with fewer steps. Most bots can't run GPU workloads. Some do, and that's fine.
My hardware is not yours to galavant in, and your fitzing around in my digital home for your server's sake just lays bare you are being self-referentially inconsistent. You value the sanctity of your hardware, not mine.
You may not realize it, but you're sounding exactly like part of the problem here.
I don't think running a fragment to calculate one blit operation isn't doing anything to the "sanctity of your machine". Believe me, Chrome abuses your GPU much more than that. Turn off JavaScript if it bothers you so much. You won't be missed.