The quality is going to be pretty bad compared to taking the composite video and sound directly at the input of the RF modulator in the console.
You say "consoles". If you're planning this as a product you need to remember that the frequency accuracy of those cheap modulators they used was never great - fine tuning to match the exact frequency (as well as the specific UHF channel of course) would be needed.
It would be as well to check, maybe using the 'bits scavenged from the VCR', how well the quality of an RF solution would hold up on a modern high resolution digital TV. Analogue CRT TVs were a lot more tolerant of noise and artefacts than digital ones.
I don't want to be a damper, but modifying consoles might be a better strategy.