Since you are using serial, why don't you use the high-potential side to pulse an LED with the serial pulses, run a short optical fiber, then convert back to serial electrical signal on the low-potential side?

You can get as much distance as necessary for insulation/isolation.

There are some mechanically coupled optical fibers with almost 1/8" diameter, made of clear plastic. The emiter is in a package that accepts the 'light-pipe', and screws tight around it, and the detector is the same.

Since the whole thing is sealed, it is insenstitive to ambient light, so no modulation is needed...

Just a thought.

Brian