Just checking your doing it right...
1. Get online
2. After "connecting to online services" message has gone, start a custom game.
3. Inject the menu
4. Start the custom game (probably on your own)
5. Enable force host in your menu. This is so it will force host in a public match...since you made the custom game, you should be host in your private game and the menu should therefore work.
6. After you have activated force host, back out and go into a public game. If your force host worked, you wont join a game, people will join into your lobby...can take a while to get the min amount of players.
7. Dont know if you need to, but I inject again in the public lobby whilst waiting for players.
8. Once the game eventually starts, you should be host and menu should work.