marking old branches as no longer maintained

I'd like to add a feature to the buildfarm that automatically picks up 
any new branch and automatically stops building any branch we're not 
maintaining any more. To do the latter, I need some way for the client 
to detect that we are or aren't interested in a branch. What I'd like to 
do is add a file to the old back branches (say from 7.4 to 8.1 currently 
- I can grandfather the rest) called "end-of-life-reached" or some such, 
with some appropriate text. As a branch reaches its end of life, i.e. 
right before the last release we make, we should add that file to the 
branch.

I think this would possibly be useful anyway, regardless of buildfarm 
utility - I still hear of people mistakenly grabbing and building 
releases past EOL, and maybe this will give one or two the extra clue 
they need that this is less than a good idea.

Comments?

cheers

andrew