If your PVM has BNC loop-outs, you could run the analogue video into the PVM, then run the loop-outs (if it outputs TTL, with sync attenuated!) to AV1 on the OSSC, then the OSSC into your capture device. If your PVM does RGB but not YPbPr, you could get one of Mike Chi’s RGB2COMP converters.
If, instead, you have a newer gscartsw (one that can forward RGsB/YPbPr), you can route one output to the PVM and the other to the OSSC.
If you don’t have either of those, you could run your consoles through your OSSC, then use an HDMI splitter, with one output going to your capture device and the other going to an HDMI to VGA converter (Portta makes good ones) going into an RGB interface (like Extron’s; to convert RGBHV to RGBS) going into your PVM.