The 3rd-party purge utility will do exactly what you seek:
The purge tool is a kind of magnifying
glass into your squid-2 cache. You can
use purge to have a look at what URLs
are stored in which file within your
cache. The purge tool can also be used
to release objects which URLs match
user specified regular expressions. A
more troublesome feature is the
ability to remove files squid does not
seem to know about any longer.
For our accelerating (reverse) proxy, I use a config like this:
purge -c /etc/squid/squid.conf -p localhost:80 -P0 -se 'http://www.mysite.com/'
-P0 will show the list of URLs but not remove them; change it to -P1 to send PURGE to the cache, as you do in your example.