As Wulfos said,

Have ALL characters (main included) set the Main for the /focus

Have some keys (F1, F2, F3, etc for example) change focus on ALL characters, including Main.

F1:
/focus [targetexact={char1}]

F2:
/focus [targetexact={char2}]

etc....

This might make you have to rethink macros on your main account, but it is probably worth it to have the option to control any char as the "Main" at any time.