It's been done, but the results are pretty far from perfect, and entirely NOT worthwhile IMO.
There's that one company who makes component cables for the SNES. Can't recall the website at the moment, but it's been posted here before, I do know that.
If you're saying that, you haven't done it yourself.
I did it to my original S-ENC SNES and the results are fantastic. Better than RGB from the same model SNES (the early SNES's have very soft RGB).
I did not use the circuit in the OP with a transistor. I simply connected Pr and Pb with an inline resistor and capacitor on each one (I don't recall the values but I also didn't try multiple combinations so my values may not be the best). The Y is already available from the multi-out. I removed the RF encoder and put a TRRS jack in its place, and I use a YPrPb dongle that are easy to find online.
Note: The picture will still be 240p and look like garbage on LCD TVs. Use a CRT or XRGB, although I have seen a lot of sync problems with component on XRGB.