I think this thread has got away fro the issue Jon wanted to discuss. The main point of the question was to do with getting the R32 4 pots on and the rotrs and drum handbake setup and would it fit, not the benefits or otherwise of it which is his business.
I've seen pics of an R32 backing plate bolted up to late DR30 style trailing arms. The backing plate appeared to bolt straight up to the mounting holes of the original backing plate. Also the 5 stud stub axles from a Z31 had been slotted in, (not that you asked about a 5 stud conversion). Therefore the stub axles from the HR31 and the backing plate and drum brakes from either HR31 or R32 etc should work.
So just use your DR30 arms and swap over the backing plate and drum brake, and if you wnat to stay 4 stud you may have to get some hub centric spacers machined up to slip over the hub to space the rotor out 10mm as the R32.HR31 rotors are 10mm thicker than the solid DR30 ones. A mate is doing this this on his trailing on an IRS Gazelle.