Well, surprise, surprise, my K100 that's been lying for 19 years has a fuel pump that didn't....pump.
The inside of the tank looked cleaner than a nuns' joke book, so I was reluctant to immediately assume a goosed pump. I did the round robin of checking relays/fuses/connectors etc.
In the end I decided to pull the pump out of the tank, applied a direct squirt of 12V, and lo and behold it was siezed.
Upon checking the plethora of information regarding replacement pumps, I thought I might tear it down and see what was up with the thing.
Popped off the crimped connectors, and so far so good. The pump looked OK. Terminals, motor brushes and wiring looked brand new.
Further disassembly revealed the 'bottom end' to be full of gunk/crap/tarnish/rust.
The shaft turns an assembly at the base which rotates by means of 5 small roller bearings, which were covered in crud, preventing any rotation of said shaft. There are two 'plates' which were similarly gunked up.
Cleaned everything up, reassembled, et voila, one fully working pump!
If anyone has the problem of the fuel pump packing it all in after sitting for a while, it might be worthwhile to tear it down and see if you can't get it going again. Cleanliness is key on this one, and make sure and use the lightest smear of fully-synth oil on reassembly. I mean light.
I've yet to test the pressure output, but I've no reason to assume it's anything other than hearty. It certainly whirred like a good 'un.
Hope it might offer solace to someone in a similar predicament.