Ok, this is getting complicated and I've never been good at Javascript. But you need to "chain" the selects on the add game view. Once the venue is selected, you need to get the venue ID passed into the admin_get_divisions_teams() function in the TeamsController. Then you need to change the view admin_get_division_teams.ctp and get the venue home team ID passed into that and then set it to SELECTED for the right team in the array.
This is complicated! I know, sorry! I've done a bunch of googling and I can't find an easy way to do this