The biggest challenge of using a strut suspension at the rear is that - completely unlike the original Jag IRS which is obviously designed to be low and flat across the top - it fundamentally requires two towers to accommodate the struts.
...- you could cut the turret off another car (like a Subaru - or almost any car with struts) and weld it into position...
This is true, but those towers (or turrets) would go through the existing structure, and into the interior.
I'm not clear on the rules yet but am working on the assumption I cant cut and move the chassis rails.
This could be one problem with fitting in strut towers, depending on frame rail spacing and strut spacing.
The other problem is what strut towers would run into in the interior, but I noticed these comments:
The transmission tunnel will be housing four modules, plus another two stacks of four on the front suspension. Plus four on the shelf above the IRS...
There us heaps and heaps of space for struts...
I've driven an XJ6 (I think a Series III, rather than this Series I), but I didn't look behind the rear seat and I don't recall the details of the trunk. It sounded at first like this body had a feature which was common in the 1960's, in which the trunk is behind base of the rear window, the rear seat back near the leading edge of the rear window, and in between them is a wasteland of awkward "parcel shelf" over the rear axle and suspension, within the passenger compartment volume. Looking at online images of bare XJ body shells, it seems more likely that these are references to the forward section of the trunk, where the floor is stepped up over the rear axle and suspension, ahead of the trunk opening and below the rear window.
(an old bodyshell image is attached to the end of this post)
In either case, that's a great battery location for some cars (as long as you package the modules safely), and if strut towers (for MacPherson struts or just springs and shocks of another IRS design) were to poke into there it wouldn't interfere with seating space or the main trunk space (although of course now battery and suspension are competing for space). There are just those structural issues to consider... struts outboard of those frame rails (visible in the bodyshell image) would require a very wide track.
While looking for information on the XJ body, I discovered that if you have buckets of money Jaguar Classic
might still produce a complete new bodyshell for you (although only for the Series III, not earlier).