Keyclone does have a hotkey for mouse broadcasting.

You push the hotkey and click on the main.
It will switch from character to character, clicking in the same relative position on the screen for each character.
The switch is as fast as a normal KC swap, so 1-3 seconds for each character.
During which time, you cannot really do much else.

In practice, you would push a macro to sync up your views as much as possible.
This same macro would cast whatever spell.
Which then has each toon need to position the target circle and click it.

You then press your KC mouse broadcast key.
And click on the first screen.
It will do the swap-click, swap-click, until each have clicked.

Play around with this on something non-critical.
See if you like it.
I found manually moving the mouse to the slave regions and clicking manually was faster.
But eventually moved to a different boxing solution, partially because of the mouse stuff.