You see “linear” break cables for sale on BMX websites and the like, seems that they don’t bind when doing bar spins etc. So what’s different about them in terms of the way they’re made? Can they be used for normal caliper/v-brakes, etc?

I need some brightly coloured cable outers for one of the kids’ bikes and BMX jobbies seem to fit the bill apart from this “linear” stuff

Er…? Probably, Yes.
I use brightly coloured BMX cables for the BB7s on my HT.

They’ll be fine. Difference is they are not spiral wound like standard brake cable outer but the internal wires run linear like gear outer (only slightly thicker). I’ve used them as gear outer too with no problems at all, as the colours are much nicer 8)

i use linear brake cables as gear cables as well with no problems and they come in all sorts of colours