hey does anyone know if its illegal to run the majority of the wiring harness on the cabin side of the firewall?

ie lots of hidden wiring and compononts?
but it is done by many, should be okay as long as it's done properly on most new cars there is a sh!tload of wiring under the dash and not too much in the engine bay. i don't see any big problems you could have with the wiring, maybe try and hide as much as possible and the rest use that wire tidy stuff the black tubing that has a split down it. paint it to match the car or leav it black the same as the car will be and you should be fine. i think they would only have a problem if there were wires hanging down in the cabin.
can anyone else verify this?

It wouldn't matter - inside or out.

Whatever you prefer, I don't see any reason for not being allowed to run it inside.

Fit a little fire extinguisher inside just to shut them up if need be.

dewiring your engine bay is fine for a show car but, if you do it fault finding becomes a nightmare, when something goes wrong.