I failed to find any way to do this via the internets; but gdb, and some kindly soul inside Apple, met the challenge:
(gdb) b -[NSUserDefaults objectForKey:]
Breakpoint 17 at 0x94172fa4
... invoke service, hit breakpoint ...
(gdb) po *(id *)($fp + 16)
Is there a better way to do this? Google and mdls don't seem to know about NSShouldActivateForServiceRequest at all, so I'm guessing not.