Thanks for all the suggestions.
Before I even posted the video I checked every bolt, nut, and screw I could find on the bike and tightened them down. I've read enough of these posts to know that these bolts are not to be trusted. Hell, I've even lost a few myself- these single thumpers just shake 'em loose.
So tonight I started up the B5 in hopes of targeting the source of the noise a little better. Wouldn't you know, the sound is completely gone. I even took her out for a few miles to make sure it wasn't a fluke, and she's thumping along just like old times. Good news for me- hopefully it was just one of those weird things that left just as quickly as it came and will never show itself again.
One one hand it's not very intellectually gratifying that I couldn't get to the bottom of the noise, but I'll take that disappointment over the disappointment from expensive repairs and lost bike time. If anything else turns up, I'll keep you posted.