I think it would be an imposition to pick his brains when there's no likelihood I will use his services. Maybe I'm over-polite, but I wouldn't do that. In the interim, my repair guy has replied to me:
I've done a bit of digging, and according to some technicians, the glue that holds the stators to the latticework becomes brittle with age, and transport/shipping can jar it loose, though it can also fail by other means. This ties in with your findings about increased issues after an active session. The pops are the diaphragm contacting the stator(s) and shorting the HV.
I pulled the old panel out of the Archives for closer investigation. Stripping it down revealed that the position of the arc coincided with a sagging stator front and back, and the glue is indeed brittle. I can easily see vibration (from music or transport) breaking more of it free. The panel we replaced last time has a totalled diaphragm (a sizable hole burned right through). I don't know whether the absence (thus far) of full-time sizzle in this lower panel indicates that the diaphragm is not yet punctured. If it isn't punctured, it is possible that re-gluing the stator may resolve the issue, but I've only got a foggy shape of a plan for how to do that at this stage. If the diaphragm is punctured, it may not be feasible.
I don't believe I have the capability to rebuild damaged panels at this stage, though there are outfits that offer such services (SDS Audio Labs, for example). There are also kits available for the DIYer (such as from ER Audio), but they require a lot of space and the time required for me to undertake that work would likely push the repair into the 'uneconomical' territory. Also, I get worried just looking at the details of what's involved (one wrong move and it's back to square one again). At this stage, I can replace panels, but I can't repair them.
It is a valid hypothesis that shipping/transporting the unit(s) will make the issue worse - these things are fragile even when new - and I'm worried about the same if they were to visit me again. I don't, as a rule, do house-calls anymore, but this may be an extenuating circumstance and I'd be willing to look into that option for you.
I wish I could provide an easy, cheap, and surefire solution, but I don't think that one exists for this. I was hoping you just had that one bad panel that didn't get enough glue during manufacturing (or something). However, it's looking like a systemic or time-dependent issue with these units (or this one unit's panels, if it's the same speaker as before).
I don't think there was an issue related to transport: the first panel failed when the speakers had lived in one spot for 15 years. The second failed after two months back at home.
So I plan to let him replace this panel (it is the same speaker, but a different panel, so not an issue with the quality of his work). But when the next one goes, I guess I have to decide whether I own a pair of expensive paperweights, or whether I replace all remaining ten panels. I don't mind an excuse to visit him, despite the long drive, as he is a fascinating character as well as a skilled speaker and amplifier repair man. If we lived closer I'd be delighted to be his friend.