Banned. Until December 31, 9999. Not just the XBL gamertag, but the console as well.
So, yes: it is possible to override the per-gamertag restriction on closed test executables like the Halo 3 Epsilon test if you can get access to the gamertag account of an authorized user.
It is also very, very easy for Microsoft to find you. So don't bother.
UPDATE: It is worth noting that this entire process is very similar to that surrounding the COD4 Beta. News outlets have been claiming that Microsoft recently banned over 20,000 XBL accounts for unauthorized access to this beta, but Major Nelson is denying this, despite admitting that unauthorized access to such a beta is a violation of the Terms of Service.
Also, this dialogue between | Scar | and another banned Epsilon player on the Xbox.com forum is nothing short of hilarious.