I was the one getting into HDR for vector games something like 6 years ago now. Aaron suggested the first thing I do is get, uh, what was it? It was to modify a device in the vector games that wasn't behaving like a scheduled CPU. I had to get that to behave like a scheduled CPU first, to properly time how long a vector was actually firing at a spot? Something like that. I bet if one searches for my last commit, it'll be me reverting the code that I did for that. From what I remember, the code was good enough, but tempest had slightly different timings and I was moving countries the next week, so it was much easier to revert my changes than wait for whenever I had time to explore if the timing shifts were expected or not.
After that was done, MAME would need to write into a frame buffer that has dynamic range larger than a byte-per-color (float would be fun! half? whatever modern graphics APIs want for HDR?). Then profit.