There was some discussion about this about a month ago. What you can do is a transparent focus based system. When you switch your window in ISBoxer, you have the ability to perform a keymap. My keymap has things like telling Jamba that this toon is now the leader. What you could do is have that keymap tell the other toons that it is the focus target they should be using for targeting and following. This is different than the typical focus based system since you don't need to push a key to do it, the magic of ISBoxer will do it for you.

If you don't want to use up your focus slot in WoW, then there are ways you can do it using ATGs in ISBoxer. There are a few creative solutions out there, but knowing you're venturing into 10 boxing, I knew your issue even before I read your post - it's your FTL modifiers like Ualaa said