If you want to be able to emergency Cover a healer, what about this macro. First, set your party list to auto-sort and put healers up top. That way you know that healers will be in the 2nd slot for 4-man dungeons, and 2nd and 3rd slots for 8-man dungeons.

8-man:
/micon cover
/ac "Cover" <2>
/ac "Cover" <3>

4-man:
/micon cover
/ac "Cover" <2>

That'll try to cover the healer in a four man dungeon, and in an eight man, it'll try to cast cover on both healers. If one is in range, it'll work on that one. If both are in range, the healer sorted to the top gets it.

Alternatively, you can put in your Focus target if you want to be able to Focus a healer to watch their HP more closely. In that case, I think it's:

8-man:
/micon cover
/ac "Cover" <f>
/ac "Cover" <2>
/ac "Cover" <3>

Only downside with this method is that you can't tell the person they're being covered, as you're not entirely sure which person wound up with it.