Doesn't
help if you only have 3 x BNC, of
course; you'd at least have to construct a sync separator, but that's
potentially just a one-chip stripboard design.
I'm not sure how easy a sync
separator is. I seem to remember there's a
SAA-family chip that'll do it, but I'm not sure how obtainable those are
these
days.
LM1881. 8 pin chip, can be found everywhere, uses a cap and a resistor
if I'm not mistaken.