custom robots.txt and favicon.ico

Im in the process of setting up a wpmu system… its basically a bunch of my sites and client sites maintained by me… which all have a completely different look / feel / functionality. WPMU just means i dont have to upgrade 20 different wp installations.

Everysite needs to have their own favicon.ico and robots.txt

Now I understand that this can be done on a theme level for favicon, but what about robots.txt?

Surely im not inventing this issue, and someone has already resolved it somehow? Ideally im after a solution that doesnt involve me having to map every file with rewrite rules based on the HOST – Im already using domain mapper, so ideally, the solution would use that.

Any thoughts?

P.S: Im very techie with full access to the server, so no problem if the solution is a http.conf edit etc.