You guy's might find some of this interesting:
http://forums.justcommodores.com.au/v6-development-modification/78409-line-locker-setup-abs.html
(Using the ABS solenoids to implement a line lock.)
I don't have the car to try it (with ABS and cheap parts replacement if it all goes wrong) so haven't done it (yet) but I still think it's a good idea.
Some corrections though:
VR and VS are still 3 channel, VT- are 4 channnel.
There are separate valves for dump and hold in VS and I am pretty sure also for VT-.
I would use an Arduino for control rather than a motor speed controller if doing it today.
I would still measure and emulate the output signals using that.
You can't do "roll control" i.e. lock the brakes on because there are one way release valves in the the blocking solenoid valves that release brake pressure from the wheel cylinders when pedal pressure is released, regardless of valve position.
I'd check the temperature of the solenoid valves if left powered for longer than an ABS stop. I think they'll be OK because the initial current spike is much higher than the hold current.
In addition to a line lock, you could actuate the solenoids for the front wheels and create a foot operated "handbrake" for handbrake turns (motorkhana). You could also use that while braking and alter brake balance - with care; you could make big changes.
If you have a VT- and 4 cahnnel you could emulate an LSD with speed measurement across the diff..