Tried ze_callLater just now, and it kinda works i guess.
But the issues you mentioned are likely to surface.
Sadly i have no idea how to hook into that, i basically have close to no LUA knowledge
The ideal situation would be to print screen when i see those things though "Quest Completed, Achievement Completed" etc, cause it's kinda pointless to print screen Before the message as the picture would just be any picture with no clue what the reason for it is.
EDIT:
Is it even possible to hook the announcements through lua?
I am not sure how it all works but as i can't see it being allowed in the addon system,
if that's the cause and hooking through external code is the option then it's surely out of hand.
Think one could bring this up to Zenimax, seems like a handy thing but i guess it's in the minority though;(