Ugh.  Hate to reply to my own post, but hopefully I 
will prevent someone else from making a fool of 
themselves.

I hadn't found the Filter FAQ - if you prefix your 
filters with
-*
it seems to take care of the problem.  The log report 
at the end was a little bizarre; it said it had 
written ~200 files for a total of 2MB, but had scanned 
~30MB, which was more appropriate.  Sure enough, the 
mirror directory is 30MB in size.  So it did retrieve 
everything it was supposed to, without needlessly 
traversing the upper levels, but the log file seems 
out of sync with what it did.

Regardless, I was able to solve my problem.