Update: I said fuck it, and also #yolo.
We put the car back up on stands last night and I pulled the sump and oil pump. The o-rings were fine. We installed the oil pump from Paul's old motor, which was worn but known to work. Paul ordered a new pump but I wanted this car running so he could at least use it while the new one ships from the States.
I tore down the oil pump I had originally used, and found it had eaten itself. Not good! Not sure why either, I gobbed it full of assembly lube when I re-assembled it. It's possible some debris got into it somehow. We found a couple of shards of metal, though I think they are from where the drive gears have eaten the machined faces of the pump.
@jared244 suggested packing unprimed pumps with Vaseline, which I think is a much better way to do it, and how I'll do it in future.
I removed the pan by lifting the engine off the cross-member about 6 inches, then removing all the bolts. With that, the pan drops some, and I put a spanner in the side to disconnect the oil pump, and bring the whole lot out together. The flaw with that approach is that it's virtually impossible to reverse the process, as it takes quite a bit of force to seat the oil delivery tube properly. But, with the oil pump in-situ, it's not possible to refit the oil pan (unless you drop the cross-member, which I didn't want to do).
So, using a Dremel, I notched out the baffling in the sump enough that I could fit the sump and turn it into position.
With everything buttoned back up (and a Kevlar timing belt that
@Philia_Bear chucked in) we primed the pump and started the car, and it was all good in the hood, giving a consistent 1.5 bar, which is low, but at hot idle it doesn't drop away, so I'm calling that good enough until the new pump arrives.
Paul is now happily cruising back down to Canberra and, apart from a wrong-length kickdown cable that is making the car hold gears an awkwardly long time, everything seems to be running A-OK