Prepared has played his army, with IS Boxer across six machines.
Not sure on his exact setup, but he's had a few posts here and there (dual-boxing and isboxer.com) detailing his setup at some point.

As Ughmahedhurtz has said.. you could go with just the mouse broadcasting mapped key.
Take a look at the Repeater/Broadcast mapped key, generated by the wizard.
Separate that, so its only the mouse broadcasting, rather than keys too.
Create a two step mapped key, that executes on press or release.
Have the first step enable the broadcasting of the mouse.
And the second step disables the broadcasting.
When you push your key (Tab or whatever), broadcasting is on... and when you let go, it turns off.

I have numerous mapped keys... that work like this (for the mouse stuff).
I push the keybind, which might be Consecrate (without any mouse stuff on a Paladin), on say Keybind: 4.
But 4 also does a "Send Next Click action" for my Death Knights.
Meaning the mouse broadcasting is enabled for their windows, just by pressing 4...
KVM/Ignore local click, means whatever window I'm in, is not going to click on anything as I move my mouse around.
Once I make that click, the targeted AoEs go off, for any toons in the group (Action Target Group) that is set as the target of the "Send Next Click".
At which point, mouse broadcasting is disabled for those windows.
So... just push 4... move the mouse (and watch the mouse targeting circles move in the DKs regions), click and its done.

Mouse broadcasting across one PC is very accurate, and switches between regions is instant.
If you're playing across multiple PC's, you won't be able to switch which region is on a given monitor...
But, you can still do the broadcasting of keys or the mouse, or both.

IS Boxer has two broadcast modes, essentially.
One is Broadcast/Repeater, which is straight broadcast of keys and or the mouse.
Another is Keymaps, which are configured actions with triggers (often a hot key combination).
If both are on, Keymaps supercede broadcasting.
You can toggle them independently.
I have Repeater/Broadcast on, and Keymaps off, for entering my password.
I play the game with Keymaps on and Repeater off.
I turn off both, to chat with the guild.
I enable Repeater without disabling Keymaps, when I want everyone to click on a portal or to set their talents.
It is extremely flexible.