I just solved this problem in my setup, with the DVDO VP30. NES and 3DO are the last consoles I haven’t converted to RGB output and probably won’t, although I did do the 240p mode on the 3DO.
DVDO VP30 YPbPr out -> OSSC AV2 in
Routing audio was a little tricky due to the VP30 not having a analog audio output. Digital audio is only active in conjuction with digital inputs, while analog audio input gets injected into the digital video output…but again, no available analog audio output. Just be forewarned, if pursuing this as a solution, you will have to solve for routing audio into the OSSC.
Anyway, below are some screen grabs from both consoles. It’s not jaw-dropping like RGB directly into the OSSC, but it looks pretty good for what it is, and in the case of the 3DO, because it is progressive, it has very little lag. And you also get the benefit of being able to enable scanlines. In both cases, the DVDO VP30 is outputting 720x480p.
This obviously doesn’t solve for RF, but maybe helps get a little closer to a solution.
3DO SSF2T – https://klovimg.com/images/2018/04/18/ST_Scanlines.png
3DO Road Rash w/ Scanlines – https://klovimg.com/images/2018/04/18/Road-Rash_scanlines.png
3DO Road Rash – no scalines – https://klovimg.com/images/2018/04/18/Road-Rash_no-scanlines.png
NES – Castlevania – no scanlines – https://klovimg.com/images/2018/04/18/Castlevania_no-scanlines.png
NES – Castlevania – Scanlines – https://klovimg.com/images/2018/04/18/Castlevania_scanlines.png