On the rear bumper of the FL they have red markings on the bottom outer edges, could it be possible to fit LED strips or lamps in place instead

The FL rear lights don't have reflectors built into them. The red strip is the reflector.

This means that people who retro-fit FL rear lights into a Pre-FL rear end don't have reflectors.

About running red LEDs in the back bumper, I'm not sure that would be legal?

You would have to be VERY careful about fitting red LED's here, I dont believe it would be illegal, but they definately could NOT be brighter than the rear lights.

Sure someone will be along soon to verify statement one way or the other

Not sure why you'd want to do that to be honest. The reflectors at that position are meant to be passive, i.e. they reflect when light obviously passes over them. I wouldn't want lighting being projected from that point all the time, however subtle or dim it might be.

And on the FL light fitting on a pre-FL, the reflector issue is only an issue on non-vRS models just to be clear, as the vRS bumper has always had reflectors built into it.