You are mostly right about 4:4:4 being in the HD arena, as it is one of the only sources that are actually recorded in 4:4:4 in the first place, so obviously you want to keep it that way when working with it. That's why I mentioned the 4:2:2 solutions that are available to consumers (via rental anyway) that can take an RGB input from a good quality laserdisc player. That would be your best bet, to get it bumped onto digital betacam or similar, and then use that digital stream as your master. You will actually end up with a *better* image than anyone is seeing out of their Laserdisc players that only have S-Video (in all its colour crawling glory)
Your next best option would be a board from someone like a targa, and use it with a lossless realtime codec like huffyuv or similar, and once again come in via RGB from an LD player that supports it. You then want to pull the PCM track off separately and keep it in the digital realm.