I do understand what you're saying about impedance mismatches and I
can also see where marrying this cable with some (probably) Chinese
baluns which are "supposed" to be constructed correctly could upset
impedances, but baseband video was what I was referring to. So in
that sense the response demands of the cable and the baluns would be
limited to 4 or at best 5MHz (right?), and then in theory wouldn't even
CAT3 be overkill?

Probably. But CAT5 or CAT5e aren't horribly expensive, nor do you need a lot.
I understand you don't want to waste materials you already have, but why not
just buy the matching type, and that will be one less thing to worry about?