Sorry to be really picky, but I just tried something that amounts to a forced error - unchecking all speakers. That puts up a modal dialog that will cause problems.

This only happens if the computer is the last speaker to be deselected. iTunes' own logic will revert to the computer as default if one of the other speakers is deselected last.

I think this is going to get tricky as I assume you are using GUI scripting and will not know the selected state of any one of the speakers - nor will you be able to maintain details of what you have done with them as there's no guarantee that you are the only controller of them.