any of the sellers of vehicles on rally specific websites like "ben's rally page" etc. ? you may find more people who've built true rally cars there than here.
lots of sellers of competition cars there, someone must have done this.
I know what you mean about more freedom w/ wiring. no amenities & non performance wiring. why worry about smog, rear defrosters and A/C?
maybe you could just plumb the harness through one of the other firewall openings closer to the LHD driver's seat & lengthen the wiring on the engine bay side? fewer wires on that end; and more segregated so they're easeir to look at & keep track of?