As a prototype, this is neat.
SCART absolutely isn’t intuitive. I don’t think there are any simple, passive solutions for getting YPbPr in and out of SCART; you could use two of these to get YPbPr in and out, but the result is going to be a chunky mess (no idea if those adapters are going to affect the video signal), and the alternative is $70 to $90 in custom cabling.
I imagine a production version would consist of a 3x RCA (GBR coloring), SMT versions of those resistors, and, if they exist, 3x through-hole male RCA connectors, so the unit can just be mated to the RT2X. (Failing that, a short 3x RCA dongle, at which point you might as well add stereo RCA inputs and a longer audio dongle so people don’t have to stretch their cables.) Bonus if the PCB blocks the composite and S-Video inputs, since those shouldn’t be getting used while you’re using YPbPr anyway.