when you pull the spark plug cap off and it's guts stay attached to the spark plug... I've coined a new term: Induced Maintenance. It's inversely related to Routine Maintenance. When everything is running right, do something to mess it up
In preparation for adjusting valves, points, etc., the BERU 5KΩ cap disassembled itself. There was a slight amount of moisture under the rubber sleeve around the HT lead, but there was far more corrosion than it would have generated ... so this must have been going on for some time.
Here's what I found after I dug all of the oxidized material out of the housing. You'll have to use your imagination (phone camera); the steel "screw" on the right  is what goes into the HT lead from the coil; the cylindrical ceramic resistor slug; and the brass ferrule that clips onto the spark plug. The interior of the metal/plastic housing around the screw was full of flaked corroded material.
This is how it all cleaned up. I also discovered that there was wee little spring lurking in the debris (the circular item is a broken off piece of it) the clip to the left of the ferrule snaps into the groove on the ferrule and retains the spark plug end.
What isn't visible on the screw is that it actually has a hexagonal shape that engages a similarly shaped recess (inside the plug housing) which keeps the screw from rotating when you screw it onto the HT lead. Due to the heavy corrosion on the screw, that hexagonal shape is now a very loose fit into the plastic (rather than a press fit).
As much corrosion as there was on the steel screw, I'm surprised that that cylinder was even running ... but it was, and well. The spark plug color is a pleasant toasty brown. It's a happy bike.
So, with that in mind I'm wondering if I can rebuild this cap; it couldn't run worse than it is now. A new cap is only $12, but it's Monday, there isn't a BMW shop within one hundred fifty miles of me (there isn't a motorcycle shop in the continental United States that is open on Mondays) ... and it would be a week before I can get one here via UPS.
Everything cleaned up well, even the little spring. The only issue is the now sloppy fit of the parts into the plastic housing. I don't know what the operating temperature of these caps is, but am thinking that RED Loctite might suffice as a "glue" to secure the brass ferrule and steel screw into the housing.
Your thoughts gentlemen? NAPA? PepBoys?
p.s. Anyone have an infra-red thermometer that they could take a reading off of their the spark plug cap with?