I'd imagine HKN would be similar.

You need somewhere where you have your toon names.
Vuhdo can do it as a single addon, but Healbot or Grid (with GridCustomLayouts) + Clique works fine too.
You want it alphabetical, so its the same on every screen.
You also want them to be positioned as close to exactly the same as possible.

With InnerSpace (ISBoxer Toolkit) you set up a mouse repeating region.
There's a hotkey you set to bring up the GUI in game, Zanthor's post suggested Alt Ctrl /.

Drag the repeater region so it covers the entire Grid frame.
Any click you make in this region will be sent to the client(s) of your choice.

Using Vuhdo, Open RDX, Clique etc, set what happens on a Left Click, Right Click, Middle Click, Alt + Left Click etc.

With InnerSpace, you can check KVM mode on the repeater GUI, so a click in the repeater region is broadcast but doesn't change targeting on the toon that does the click.

Once the region is set up, it basically turns mouse repeating on, but only when you're within that specific region. At the same time, it only sends the clicks (and modifiers) to the clients you have selected.

From my understanding, if you wanted multiple toons to be able to broadcast their clicks, you'd need to set up regions on each.