Hook "chooseteam" command ?
You can also hook some of other player's default commands... you can see commands by testing them on your client, just type "bind key" and you'll see the command, if it has a + in front, it's most likely it can't be hooked, but be sure you're testing keys on a DEFAULT layout.
Also have a look into hlsdk_const.inc at IN_* defines, those are actions, like IN_SCORE is when player is holding scoreboard button, you can check those against get_user_button() with &... if(get_user_button(id) & IN_ATTACK) and you should do that in FM_CmdStart.