Right, after a certain degree of head scratching and chasing my own tail, I eventually managed to kick the Mac Pro actually boot from an installation medium.
Despite having tried a few things which should work, I couldn't convince it to boot from a USB device. Playing around with the partition scheme appears to have bricked one of the thumb drives too...will faff around with that later...at least it's only a 4gb freebie - if it was one of the 32gb ones I would have been more irked.
One idea which as far as I am aware *should* have worked was to load up an a suitable third party boot program which you can point to external devices. I have one of these by the name of Plop Boot Manager, I find it quite handy as a couple of PCs here can be really picky about booting from USB devices.
This almost worked. It booted from the CD just fine into the boot manager. However as soon as I hit enter to tell it to boot from the USB device it got precisely this far then totally hung up.
I spent longer arguing with that than I probably should have. Still don't know why it wouldn't work. By all rights it should have.
Knowing that the thing would boot from the optical drive though, I figured I could just throw the Mint installer on a DVD. Only reason I didn't do that initially was that I didn't want to spend half an hour trying to work out where the blank DVDs were.
However this didn't work. It threw up a somewhat cryptic menu asking me to "select the boot CD type" - but apparently giving me no way to select anything.
A bit of research revealed what was going on. The issue is that due to the age of the machine, the implementation of the EFI boot system is a very basic 32-bit implementation. As such it doesn't quite know what to do with the DVD I am trying to boot from, and throws that menu up. ...However (you'll like this) that menu pops up BEFORE the keyboard has been enabled, so I wouldn't be able to select an option even if I knew what I was doing.
So what you need to do is modify the bootloader on the DVD (well, modify it in the ISO before burning it to disk). That's the sort of thing I used to do back in my days of faffing around with the Amiga - but is a bit beyond my pay grade these days!
However I was very happy to find that someone has already done the hard work for me, and a whole load of ISO images worth the necessary boot loader modifications done - they're over here. Worth bookmarking for those of you who enjoy messing around with computers in ways other than how the designers may have planned.
Writing the modified Mint 19 installer to DVD immediately resulted in the installer starting up as I hoped. Only took me half the afternoon to get there.
Glad to report that once we got there the install proceeded by the book and we were up and running in about twenty minutes.
I've decided to manually copy stuff over from the backup as I'm treating this as a good opportunity to clean up my Home directory, there must be configuration files in there from three or four OS generations ago.
Got most stuff set up now and it's copying a couple of terabytes of videos, music and photos back over from the backup drive. Figured it can be left to do that overnight.
I did spend a bit of time on car stuff too this afternoon. Main plan was to pull the CVT pulleys off the Invacar. Yeah...turns out the bolts are highly disinterested in moving (yes I have flattened out the lock washers), and I decided to stop before I wound up breaking something.
I have left the bolts soaking in Plusgas overnight, will have another shot tomorrow. This is definitely one of the times I wish I had an impact driver as I'm sure that would shock them free in no time.
...May be an excuse to visit Toolstation tomorrow.
The CVT setup definitely hasn't been happy, the fact that the entire area around it is covered in finely atomised black rubber powder from the belt, which I would not expect after only fifty miles.