Recording only specific frames

For context, I'm looking into creating a tool-assisted speedrun mod for a modern Unity game called Rain World, which will mainly just allow it to be slowed down or played frame-by-frame. The issue is how that will be recorded, since I want it so the game can be played at a slowed speed, but recorded at normal speed.

My current idea for that is making an OBS plugin and using OBS' websocket features so my mod can talk to OBS, and tell it to only record specific frames (or drop every frame except the ones I want).

I mainly just want to know if this is a sound idea. It sounds like it could work to me, but I haven't made an OBS plugin before, and I haven't looked too far into it. If that's a bad way to do it, or if it isn't possible to do what I want like this, what might be a better idea?
 
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