A little update...
I remembered I had an old style 143mm Hope wavy rotor so I put that on the wheel and mounted the caliper. I used some electrical tape to tie the brake lever to the bar so the caliper would naturally stay in place. I then tweaked the position of the caliper to match the IS standard for the location of the bolt holes. (oddly the inside face of the dropout isnt in line with the mounting face on the caliper. Per the IS these two areas shouldbe in line. I checked the caliper and the pads are nicely centralised. Perhaps the Hope caliper is designed to run quite thick wahsers on the mount? I don't know.)
Having positioned the caliper I then set about modding the A2Z mount. Its a very tricky process and involved taking small amounts of material off at a time and constancy checking the fit every 5 mins or so. I had to file the area in 3 stages, each with a slightly shallower angle to match the taper of the satstay (the angle of the seatstay necessitates this). A really worrying process that involved several cig brakes to calm the nerves!
Once I got into it I developed a technique for getting a good shape to the contact area. Its not perfect but its pretty good I think for a hand tool job.
A member over on the MTBR frame building forum is sending me some scraps of tubing for the brace. Having said that some on the frame building forum have told me its no really needed. Part of me wants to leave it to save weight but the other part of wants to hae it there for peace of mind. I'll probably have the brace but I'll make it as small as possible. Still not 100% decided though.
Next up is to make a jig to hold the mount in place for welding. Then I'll buy a big packet of biscuits and find a friendly local welder!