Re-did the XP Pro SP3 CD just in case and lo and behold it works, but under "Removable Devices" not USB Devices.
That option failed to work or even register as connected before. So it would seem these options are dependant on valid media being inserted.
I had tried a bootable XP flash drive and for some unknown reason the Repair option never was offered. This way, using the CD/DVD drive via USB, it is. But remember to try Removable Devices as the boot option.
If this hadn't worked my other option might have been an Upgrade Install from within Windows but I think that would have wiped everything.