I used the Injection-Info header from Google Groups.
Example .slrn-scores rule:
[*]
Score: =-9999
%Expires:
% Subject: Re: Can you make living with Linux\?
% From: Jonas Eklundh <jonaseklundhsandman@gmail\.com>
% References: <8fece6da-1045-45f3-9f02-46022d877945@googlegroups\.com>
% Xref: aioe\.org comp\.unix\.shell:29164
% Newsgroup: comp\.unix\.shell
Injection-Info: .*account=v_2vHwoAAACkQKbseN841UEbqgaDTAQd
%EOS
The [*] means it applies to all newsgroups, I think.
All % lines are just comments; the only filter rule is
the Injection-Info: one.