I have used the hub/disc/caliper assembly as a positioning tool on a few with varying success. I then made my own jig with less success (tolerances need to be very tight and i'm not really a great machinist at this point). I just received an anvil post punk with dummy axles and expect my success rates to go up. they make really beautiful stuff.

Anyway, one problem with using the caliper as the tab holding device, aside from the inaccuracy, is the heat that goes into the caliper. with TIG the HAZ is smaller and faster so, maybe not as much a concern with TIG. I get that the calipers are designed to get hot during use.

Here's how I did it:
I used a fully functioning brake set I bought used at a local shop for $5. I installed the hub, disc and caliper on the disc. I then attached the brake lever to a bar and used a toe strap to squeeze the disc with the caliper in the right position on the disc. I tacked the mount to the stay keeping the flame as far from the caliper and disc as possible, then removed everything to stop the heat transfer. once it had cooled to the touch, I re-attached caliper/hub/disc to check alignment (make sure to clean out the hard flux so it all sits right). then uninstalled everything, added flux and finished brazing. It works but is slow and fiddly. I wouldn't use the caliper after it was used to braze the mounts. #ghettoframebuilding

Metal moves when it gets hot and if you hit the disc with the flame it could deform enough to throw this all off. The tolerances are tight - as in +-.1mm. One mount ended up off by enough that I had to machine tiny little spacers to make up the difference. Heaven help me if I ever need to rebuild that bike and lose the spacers!