Under 1.8c you'd need to override the default filter (which is not
necessarily recommended, since you'd have to provide a substitute
explicitly in the list header, with "Filter= Only,..."). Personally
I'd consider this fairly dangerous.
Under 1.8d you will be able to specify overrides to the default filter
in the list header, eg,
* Filter= Also,[log in to unmask],Allow,[log in to unmask]
(although I think allowing <any> mail from an automatic service onto a
list is not a very good idea). Note that the "Allow," part of the
filter specification MUST come at the end. This is documented in the
current 1.8d beta manuals.
Nathan