Thanks for the tip, but I still think finding a well-preserved BVM is worth the trouble, and I don't really need anything above 20 inches.
Anyway, what do you guys think of this setup:
NES (NTSC) with PAL MasterSystem power supply - composite out => composite to SCART adapter
PSone (PAL) - RGB SCART out => SCART cable
Then plug the cable and the adapter into a good SCART switch.
Plug the output into a sync cleaner (needed for the PS) like the Sync Strike with a VGA out.
Then:
option 1) Take a VGA to 5 BNC cable (like the one here: http://www.ambery.com/pr15vgato5bn.html) and plug it into a multistandard BVM/PVM.
option 2) Take an ordinary VGA cable (I think some BVMs have a VGA in) and proceed the same way.
Which one do you think is better?
Wire in any additional consoles the same way (add an RF demodulator and/or a step down transformer, if needed).