This is my system (perhaps I'll post my isboxer config, but for now you'll have to figure out on your own how I accomplished this):

F1, F2, F3, F4 switch to the applicable character.

When I press F2-4 (the Alt's F keys) broadcasting is turned OFF. This is because when I would be switching to an Alt I would either be looting or resyncing the one char or closing a menu that only that char has open and wouldn't want broadcasting.

When I press F1, I would want to turn broadcasting back ON, as this is my main. At times I would want to have broadcasting turned OFF on the main, and I have 2 keys: Accent Grave/Tilde will turn OFF broadcasting as long as it is held and turn it ON when it is released. This allows me to click around without broadcasting for small periods of time (such as looting or equipping or moving inventory). I have another (Alt-g) to toggle broadcasting on the main as well, in case I want to chat and don't want to be bothered with holding down Accent Grave.

There are probably other cases you could find to use logic behind turning broadcasting on and off. I recommend you make a Keymap specifically for Hess operations so you can make it run instead of repeating the logic for each key press you want it assigned to.

Also, as a change to the default Wizard setup in Isboxer for d3, you might want to separate mouse broadcasting from Keyboard broadcasting, and instead just use key maps for the skill keys you want broadcasted. Normally, the whole keyboard is broadcasted, but you might not want all of yours to open their inventory when you press C (actually, I do allow this, through a keymap, but I also turn off broadcasting AFTER the 'C' has been broadcasted, so my main can work with the mouse without also manipulating the Alt's inventory).

There are lots of quality of life improvements you can make for yourself.