This would have to be thought through a bit more but the SSL (Suplex) front suspension system has adjustable spring seat heights and a spacer which is used to adjust the ride height.
I think that if you spoke to Suspension Supplies Dan White or Peter Ballard, they might have an idea how you could do it with the correct combination of bump and rebound springs together with an appropriate length spacer.
I might be wrong but it's worth a phone call.
