What happens when someone sends me an iMessage/SMS during a restore?
In my experience, when I send a user an iMessage while that user is unable to receive one, my iPhone (or whatever device as long as it is sending through my phone) will switch to SMS. When the message is sent as an SMS it will still arrive even after the recipient is re-connected. 

Note: I haven't looked through docs - this is just what I have come to through observation.