I am starting to feel exceedingly incompetent...
I added dumpio to the loadmodule list, restarted httpd. There is
still no additional logging output.
I have tried a few more variations of the ReWriteCond line in
<.htaccess>. Because the total lack of any kind of response, logging,
message, etc., I get the feeling that the Rewrite* directives are
being totally ignored. Only when there is a syntax error does anything
change ("Internal Server Error").