It's not the only pound coin I've got of that sort, I'll cash them both in one day maybe.
On the left, an original saloon seat brace. On the right, a cut-down hatchback seat brace. They're similar enough in the right places that they are interchangable with some jiggery-pokery of the hatchback piece. The modified hatchback piece also bolts into the existing fixing points in the saloon body, as I hoped it would.
With the modified brackets fitted, you'd be hard pressed to see what I've done, which is perfect.
For the seat back bracket, we had a look under the car and found that two of the captive nut holes are pre-drilled in the strengthening plate which made lining up for the other two holes really easy, especially since we had the piece cut from the donor hatchback to help make sure all the holes were in the proper places. This was a pleasant surprise as this was potentially one of the more difficult bits to get right.
Once drilled out, the bolts could be dropped through from the top, bolted from below (I hadn't welded the nuts in yet, since we were trial fitting) and the seat catches aligned properly. Everything fell into place with minimal fettling, it was surprisingly easy.
The rear seat base needed two holes drilling for the captive nuts, only afterwards did we realise you can't actually get to the other side of this so instead the holes were made big enough to accept the bolt heads and they'll become captive bolts instead. Alignment on this was also easy since there's cut-outs in the sound-proofing foam in exactly the right place and the seat base sits in the saloon's seat base location perfectly, probably because my suspicion that the shell is identical up to the rear suspension turrets is correct.
I didn't get as far as welding in the captive bolts for the seat base today, and only three of the captive nuts decided they wanted to stay welded, so I'll go back and do those tomorrow probably. I also haven't fitted the seat back side pegs yet that serve to allow the seat back to fold down without going all wobbly sideways, but again, that's not too bad a job to do and they look like they'll go straight in on the car easily enough. However, enough fixing points were secured that the seat could be trial fitted and I can say with confidence that you can indeed fit a hatchback seat in a saloon body and it works rather well.