Are you wanting to run Linux ON the router or are you wanting to use a Linux computer with it? There is no custom firmware for this particular model, because for the most part the software (Streamboost support) is something unique to just this router and it's primarily the main reason you buy this router.
If you're just wanting to use a Linux computer with it that should work as long as it's wired. Wireless is a little, iffy, in my experience (if you set it to 802.11g mode only though it seems to fix wireless stability problems), but a wired Linux computer works just fine for me. Are you trying to use a Linux computer and running into problems?