At this stage, I'm thinking the best solution could be to combine Sideroad's idea for a disc/ring/plate and one I have had.
If we built a thick spacer, say 20mm thick or more to mount to the back of the hub, we could have 5 holes in it corresponding to the stud holes in the hub. If we then got longer studs, they could go through the spacer ring then through the hub to the other side. This would securely hold the spacer ring against the hub and not reduce the hub's strength in any way.
We could then have another 5 threaded holes in the spacer ring evenly spaced between the stud holes for the disc to attach to.
Then, by simply choosing a 5 hole disc which has less height than the one we have now the spacer would be made the right thickness to compensate.
Sideroad has already found a disc which may be suitable.
http://www.dba.com.au/products/dba579
We need a total height of 49mm, the plate above is 29mm high so we would make the spacer ring between the hub and the disc 20mm thick.