I think I may need a little more information, because looking at it form the imm-side of things, I don't understand why this is helpful.

If I haven't caught you on in a billion hours, then the problem isn't that I don't have a way to leave you a message, it's that I don't know you and therefore I don't have anything to say. I don't think this would help that.

If I'm impressed and need to give you something that I've already picked out (immortal experience, a title, more empowerment juice, general 'attaboy!" peptalk, etc.) I have plenty of options. I can get one of my imm buds to pull the strings for me or I can leave you a detailed note (and we have even more toys in our "note" toolbox than we used to.)

If the issue is more that you want to know we're logging on (when you're not) and feel bad about never seeing you vs. we've gotten hit by a bus and/or have no idea you exist...then that's kind of what the dormant wizlist list and emailing immortals for problems catching them in game are about. We've recently (though I'm not sure it's implemented) even discussed ways to keep the wizlist more up-to-date than it sometimes gets.