Would it be possible to make this partially work with Firefox 3.5's wonderful "Forget about this site" feature?
According to an earlier poster, the "browser:purge-domain-data" notification lets you know which domain is being cleared.
I expect this could only work for rules which match a specific domain. Basically, if a rule matches a.com or *.a.com, then doing "forget about this site" for a.com would clear hits for that rule.
This is likely to just match a limited set of domains, and might be rather difficult to implement, so I'm not sure whether it's possible or worth doing, but I thought I'd at least suggest it.