We ended up working around the issue of retro reflective MRI-compatible and MRI-visible markers. We have tried using paintball bullets and sprayed them with retro reflective spray paint. It didn't work great though. The sprayed paintballs weren't as visible to our mocap system as the official markers, but most importantly they didn't show up well on our MRI scans. This problem can probably be resolved by using fish oil capsules. Fish oil shows up very well on MRI so if you can find round capsules you could try spray painting those with retro reflective paint.

Hope this helps.