Mouseover macros or a click healing addon like clique require clicking or at least mouse manipulation. You will need to use software that allows mouse broadcasting.

The only other alternative is to set up individual hotkeys to target and heal each of your characters. Assuming you have 5 characters with a priest healer and only utilizing the most basic healing setup, you're looking at probably two ranks of a single target heal, shield, renew and prayer of healing. That's at least 21 hotkeys you'll need to find and remember.

It's much more effective to use click healing. ISBoxer has this feature and many others that are amazing for boxing any game but particularly WoW. It's well worth the small subscription cost.