Still looking for a solution for this issue. Even looked at Weakauras, but didn't find a way to add this. It's really a bummer to have to summon multiple people to raid, and have no idea who I have already summoned.
Is there a solution to show a "summon pending" icon? All other healing frame addons have this (Grid2, Vuhdo, regular Blizzard frames). Am I missing a way to add this?