Only thing you can do is run the OSSC output through another scaler like an Extron DSC series. This will normalise the signal to a standard resolution, at the cost of a frame of lag. Panasonics have amazing IQ, but absolutely terrible tolerance for anything but precise standard SD/ED/HDTV timings.
Thanks for your reply. I suppose the TV still has to upscale the image output by the OSSC anyway, so if I buy an upscaler, the upscaler will be doing that task rather than the TV, so there shouldn’t be a downside.
The only HDMI to HDMI upscaler I can find is one by Vcando. It gets good reviews, but I don’t know if it would accept the HDMI signal from the OSSC anymore than the Panasonic TV does.