Thanks for the input guys
I've 'fixed' it for now... It was the chime module under the dash, just beside the fuse panel. I really don't know why this would trigger a seat belt light, as it has wiring for the seat belt buzzer which I confirmed works any time I short that two pin seat belt connector, but I dug through wiring diagrams for an hour straight and NONE of them show the chime module having ANY interactions with the seat belt warning lamp at all! It's a head scratcher, for sure. It's also incredibly hard to get accurate wiring diagrams for these cars... I have a few different diagram books for 1995 RHD XJ's and they're all different, and none of them show a 6 pin black chime module like mine has, only the 8 pin version (Pink / Blue), so they're not that great like that.
I opened up my chime module, and honestly it's visually perfect - all windings intact, no damaged wires, no broken solder, nothing. My only guess is there must be an internal failure / break on one of the tiny windings somewhere, beneath the outer layers where it's not visible to the naked eye. I know this thing is one of the most common parts to just throw away for 99% of XJ owners, but I prefer to learn how things work, at the very least, before I do away with them. This one has me stumped as to how it's triggering the seat belt warning lamp, as no wiring diagram shows it having any interaction with anything beyond some basic ground connections, and the seat belt buzzer wiring... I'll work it out eventually, some day. Some pics of the module for those who are curious