Been busy at work so I have only done a few things. As raised by nwordjim here, my main fear was that my headstock rebuild would snap when I tightened up some strings on it, so my priority was to get it to string-up stage and see whether it broke.
Drilled pilot holes for tuners - you can see the A is slightly too close to the low E.
The piece-o-crap drill press I have been using is to blame, for anyone else contemplating headstock work - you MUST get a reasonable drill press. I got kind of depressed afterward - I didn't actually realise the pilot hole was off until I did the rest of the drilling. Another thing I found out - using a large drill, the work will get grabbed and pulled upward by the drill bit, even if you are expecting it, and even if you say "well, I'm not going to let THAT happen again." So my plan to drill 11/32" on the front and 1/4" on the back only succeeded for 4 out of 6 holes.
I marked out my bridge line (the frontmost edge of the saddles should be at the line, in case anyone's wondering) and the locations for the plate. I figured screw holes were easier to fix than post holes, so I decided to do a temp tailpiece mount by putting screws into the tailpiece bar and "floating" the plate slightly above the body. I used an old Strat copy bridge to sit in place of the Jag bridge to see how it lined up.
All I had were three used bass strings so on the basis of length I tried stringing them up randomly to put them under tension and see what happened. Basically it works, no sign of movement or stress that I can see. The strings are up to pitch, or at least in the ballpark.
Obviously only a half-load at this stage, I need some more long strings! Also at this stage - the tailpiece is way too high (I used M6 / 20mm screws, which sit up very high above the plate). The plate is also a few mm above the body because I didn't want to rout anything yet. The Strat bridge is also way too high. End result ... somewhat high action:
But I think we are well on the way now, at least with the mechanics. Now I have to figure out the body routing. I put on the plate and pick-guard as a motivator to push on, it looks pretty good I reckon:
Next steps: drill bridge posts, bump in some tiny holes for the tailpiece temp screws, and get it all down to a realistic height, then string it up for real if I can get the right length of strings.