Ok, here is my attempt at a definitive answer. Adjustable brake proportioning valves are not allowed by IVA. You may get away with an adjustable valve that is locked up in a way that it can't be adjusted once set - eg welded up.
Early non ABS sierras had a valve in the rear brake line that stopped the rears locking under heavy braking. The valve is an inertia device - it has a ball in it that closes off the port if the rate of retardation is high enough. The valve operation can be adjusted by altering the angle at which it sits - the steeper the angle the more retardation is required to make the ball travel up the valve and shut the rear brake line off.
I fitted one to my last buid (Robin Hood 2B) it passed SVA fine, I have fitted one the same to my roadster. I have not taken it through IVA yet but have confirmed that if I stamp on the brakes the fronts lock first so it should be fine.
Hope that helps, if you need to see photos and further details have a look at
www.deepfolly.co.uk
Enoch