Thanks, everybody for the useful hints and links. It turns out that I overloaded a tuning resistor rather than cooking the whole tweeter part of the crossover network.
Evidently this is a known possibility in these speakers: the crossovers are mostly "potted" (cast in resin, presumably to reduce resonances & sink heat) but these particular resistors are outside the resin. Later speaker versions even had an easy access door in the bottom of the WATT (I had to remove the whole bottom and dismount the crossover to get at 'em). And the new replacement resistors have a big heat sink clipped on.
For anyone who thinks they've blown a WATT tweeter or corssover in the future, check this resistor (value 3.2 in my speakers) before panicking: a multimeter across the leads showed zero conductance. If you show the same (you can test it in circuit) just contact your local dealer for new resistors. A $20 fix, kindly immediately sent out by Wilson (thanks, Jarron!) when requested by Goodwin's High End (Thanks, Paul!). All that's required is an intrepid willingness to venture inside your lovely speakers and the mose basic of soldering skiills (though you may need a pretty beefy soldering iron).
Again, thanks everyone!