Can't Heal You
Can't Heal You will automatically whisper people who you try to heal or buff, but who move out of your range or aren't in line of sight. That way, they know you're trying, and can possibly move to help you heal them.
It works automatically -- you just cast your healing spells and buffs the way you already do, even if that's through Healbot, Grid+Clique, or whatever. It also only whispers people who are in your raid, party, or guild, so you don't whisper random strangers you decide to try to buff.