I use Healbot on my DK tank to "display" the health status.

I have 5 regions defined over the Healthbar of Healbot, so I can click those "regions" to issue a KEYPRESS on my healer.

My tank is always partyleader and my healer always the first to be invited.

From there on my healer knows who is on which "region"

Code:
e.g. Num0:
/cast [@party1] Flash heal

num1:
/cast [@player] Flash Heal
...