Kind of sad replying to my own thread, but as no-one else has posted yet and as I've had several breakthroughs today, I thought I'd post an update:
1) USB POST loop - Plugged USB powered USB hub off EBay which arrived today into extension (in place of cheapie 4 port hub). All worked ok and appears to be booting from cold ok now.
2) USB power cable - As above (not required now)
3) Hibernate/resume issue - A bit of a Eureka moment today I think I've worked out what the M2 board was trying to achieve and why the display was dark on the first key turn... I suddenly worked out that turning the key from ignition to start counted as a 2nd pulse. Therefore all I needed to do was turn key to ignition (computer starts to boot) and wait for car self-test to complete as normal, then turn to start and release - screen then powers on and computer is part way through boot up (I hadn't tried this before because I didn't want to keep starting and stopping the engine, so was just using ignition on/off - hence the confusion!). I'm guessing here (it's not obvious from the manual) that this is an intended feature, either way, it's doing what I want it to now. Only thing is, if I want to use PC without starting engine, I need to do 2 key turns, leaving ignition/accessory on on the 2nd turn, which I can live with!
4) S3 Video driver - Managed to install Hyperion drivers so that S3 icon comes up in Systray. Still seems to lose desktop position on a restart though. Anyone know how save the settings/modify the W7/S3 ini/registry etc files so that it will remember the position?
5) Audio levels - I've coded a button in RR to give me a skin where I can access the mixer (SndVol.exe) while in RR, which is a sort of fix, allowing me to set the volume levels while in RR. Just need to see if the comp keeps them after hibernate resume cycle now (which I'm hoping it will if both RR & Garmin apps are running when it goes into hibernation)
6) AI Net live signal to keep switcher across for Satnav only use (intermittent sounds) - Still waiting for inspiration on this one...