maybe a bit out of the box thinking, but why not define macros and assign keys on the healer.

I use HKN and have the Healbot area of the party members as Click areas.

e.g. If I press right mouse on an area in Healbot where the tank is (tank is my main, so it is always the first) HKN sends a key to the Healer client (I used the numpad keys for that .)

e.g. :
left mouse in area -> healer client press num 1 ==> flash heal on tank.
right mouse in area -> healer client press num 2 ==> greater heal on tank.
...

The only drawback is that the healer needs to be party1 (from the tank), so that I only have to take into account that player and party1 are swapped. For the rest party2, party3, party4 are the same.

Maybe something like this is also possible with IS. Copying mouseclicks from one window to another is a bit sluggy on my computer so I went to keypresses, which HKN can do perfect.