My heal macros for each minion are set to heal first the minion's target, then (if there is no target) the focus. Focus is always set to my main (tank). I then have four targeting macros (one for each minion, aside from the main), and a fifth that clears the target. When I need to heal one minion specifically, I hit that macro and, since the healer(s) now have a target, the heal will cast on that rather than the tank. My tank remains free to target whatever she wants. When I'm done healing the minion, I hit the clear target macro so they can heal the tank again.

When I first set this up, before I was used to it, it screwed me a couple of times. I would forget to hit the clear target macro, which was a problem when the tank really needed a heal. I solved that problem like any raid leader would: by loudly blaming it on the healers' lack of skill.

For battlegrounds, you'll probably want to extend that to allow you to heal other targets as well. That could be done via a modifier like CTRL (if the modifier is down, heal the focus-target, otherwise do as normal), or with a regular /assist macro that sets all the minion targets to your main's.