Admittedly I haven't used it for anything nearly as "esoteric" as some of the gear Cameron has I'd be surprised if he encountered any issues. Though being open source I'd suspect if he did he could simply fix them himself in the OSSC's firmware if needed!
An OSSC will happily upscale any weird resolution/framerate you feed it to something modern HDMI devices can "understand". I use an "EVGA XR1 Lite" [2] to take the OSSC's HDMI output and feed it in over UVC to my laptop with USB-C. Works flawlessly and is dirt cheap
If Cameron wants a bit more of an "all-in-one" solution, myself and other retro tech youtubers like Tech Tangents, RetroRGB and others use old Datapath RGB E1S cards [3]. This thing is basically just a DVI port glued to an FPGA. It doesn't care about "resolution" or "refresh rate" it just DMA's raw pixel data right into RAM and lets what ever application you point at it decipher it. Personally I think they're pretty awful (comparatively) to the above option. The driver is a binary blob (so it probably won't work on Cameron's POWER machine) and it crashes a lot both under Windows and Linux. Though it seems more agreeable under Windows for changing resolution and other settings via OBS
[1] https://www.retrorgb.com/ossc.html
[2] https://www.evga.com/products/product.aspx?pn=141-U1-CB20-LR
[3] https://www.datapathltd.com/datapath-products/video-capture-...