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Plumbicon tubes inherently have unity gamma. So the cameras had 2 chances to tweak the colourimetry. WIth matrices before and after the gamma correctors. It took a great deal of work by the designers to get good results. Matching an IO for Y with 3x vidicon for RGB sounds like a nightmare.
As soon as you convert from RGB to a YUV colour space there is the possibility of illegal colours. Just as true for digits as analogue. Some years ago I designed a piece of kit for a client that took SDI in, legalised it and gave an SDI output.
Doing this crudely isn't hard. Convert to RGB, clip and convert back to YUV. This can look horrible in practice so I designed some soft clip methods and desaturation algorithms to avoid the YUV>RGB>YUV conversion. It proved surprisingly tricky to get results that were both technically correct and looked OK. One part of the algorithm needed several divisions at full (13.5MHz) video rate. FPGAs have had fast multipliers for many years but division (I think it was 26 bit numerator, 12 bit denominator) took a lot of logic. I think I used a single divider running at 54MHz to do all the divisions as separate ones would have meant a stupidly large FPGA.
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