I finally broke down and ordered one: the 6-pin legless flavor (minimal footprint), with a 0.1" socket on the other end of the cable.
I'd actually designed the 10-pin footprint into a couple of boards over a year ago, but never needed to use them; those MCUs actually get programmed using the serial-port bootloader, and I just included the JTAG option in case it might be needed eventually.
Welll... now I'm designing yet another dense gadget with an AVR, and, while I did have some other options (use an inconvenient high-density header or FFC connector; preprogram the chips using a very expensive QFN socket unique to that footprint; have the chips preprogrammed with the Arduino bootloader by a supplier), it seemed time to go the magic plug route.
Ordering directly from Tag-Connect seems to be the cheapest approach, even though they haven't made any deals for bargain shipping. While I was at it, I ordered a couple of the clips, which I probably will be using at some point.
Anyway: cable ordered; "connector" designed into new gadget. The gadget itself won't be going out to fab for a few days yet, and that'll be a 2-week turn, so figure early February before I'm actually doing anything with it. Probably have a report then.