[Exim] "Opt-in" filtering

Hullo. I am trying to implement recipient "opt-in" RBL rejection. I
have a number of RBL lists specified, all with /reject. Since we want
all recipients by defualt excepted from this, we have:

recipients_reject_except = *

Now, I can do the following:

recipients_reject_except = !user1: !user2 :*

etc, etc, but this:

1. Requires an admin to edit the exim config file (or some other
central file)
2. Would get REALLY unwieldy real fast, since we have many thousands of
users.

I could address 2 with a simple lsearch file lookup, but what I'd
REALLY like to do, is somehow permit a user adding a ".nospam" file in
his homedir, and have this somehow be detected in the
recipients_reject_except.

What would be even cooler, although I don't think its currently
possible with changes to exim itself, would be to permit each recipient
to be able to choose which RBL lists he wants to apply to any email
trying to reach him..

I'm also curious, for the default case of being excepted from the
RBL(s), does exim follow the /warn behavior, or does it behave as if
there was no RBL match at all?