Great stuff, you guys. I stopped at NAPA on the way home and bought a ... thingie. 1/4-inch, no ratchet, about 24 inches long. Breaker bar / flex something or other, whatever. Solid steel. 
We'll see if that does it. Thanks for the moral support!!
(FWIW, I love everything else about these brake assemblies - simple, effective, robust. I'm not mad at the engineers or manufacturer, just the crudded up bolt.)
1995 525iM - 131k - Alternator play, broken s-belt
1995 525iA - 147k - Tensioner weirdness, broken s-belt
2001 Intgra GS-R - 130k - spun bearing
2002 Honda Odyssey - 80k - fragged transmission