Setup healbot to display the same on all toons.
Setup healbot binds on healer only and leave all binds empty on all other toons.
Load ISBoxer and setup a repeater region over your healbot display to forward mouseclicks to healer.

Click and heal no matter who you control as if you were playing the healer.

I don't use healbot, but I use OpenRDX to accomplish exactly this and I heal with all 5 of my shamans in WinterGrasp all the time... It's AMAZING how hard it is to kill a guy with 5 riptides on him and 5 chain heals following it up...