Thanks once again for the info thesteve!
I finally did the mod today, and plumped for using an external htc charger, that has a usb to barrel lead - then in the PCE I took out the black cube thing near the socket (sorry not sure what it was) that had 4 legs - and routed the live into the fuse as per normal. Then chopped the 7805 out and linked the input and output legs together - this way the power switch still worked.
All working fine after a quick test with Military Madness and R-type
Thanks again to everyone for their help!