This is one of those things that has been done for a bit, but I guess I never released.. you guys can have it..

===========================
   HALO 3 BETA UNLOCKER
===========================
By Xenon.7(DeToX)

H3B Unlocker, an internal version of the Halo 3 Beta Map Loader by Xenon.7 and Anthony, for non-dev use.

This tool takes a clean, retail "default.xex" from Halo 3 Beta and will output 3 modified XEXs.
-h3b-highground.xex
-h3b-snowbound.xex
-h3b-valhalla.xex

Launching one of these XEX's will force a map to load when you go to the main menu and click "Quit to Dashboard", allowing you to improperly play the beta again. Note, since it forces a map to load, you are playing local, on a temporary profile, on the gametype Assault, until the timer runs out, then you can restart the XEX.. This patch is mainly for testing purposes and some modification testing.



   Features
   --------
   -Load a map when clicking "Quit to Dashboard" in the mainmenu.
   -Removed intro videos for faster loading.
   -Removed .map file checks to make modding of content possible.
   -Tool will auto detect if the XEX you opened is not a clean
    one, and will not proceed if it isn't, unless specified.

   To Do
   -----
   -Proper unlock eventually that makes the beta think it did not
    expire and unlocked custom games menu to select any map,
    gametype, etc.



========================================
HOW TO APPLY PATCHES TO THE EXECUTABLE
========================================
-Open "H3B Unlock.exe", open up a clean XEX from Halo 3 Beta, and specify which folder to output the modded executables to.
-Click apply patches.

IMPORTANT: It's also been reported you should use xextool for all 4 .dll files in the game. Otherwise the game MAY freeze on launch or give an error message upon launching.


========================================
     SPECIAL THANKS TO THE FOLLOWING
========================================
-xorloser, for his xextool.
-xbox7887, map force load talk.


=================
Happy modding!
=================
~Be sure to visit Rework3d.com for Halo modding help and details, and please give credit where credit is due.

Note: Little to no testing with the tool to apply the patches, but the patches themselves work. Enjoy.

Download:
http://xenon7.net/files/programs/h3bunlock/h3b_unlock.rar

Source code for patch to see how it was done:
http://xenon7.net/files/programs/h3bunlock/halo3_beta_hacked_src.txt