dont know if there is plans on a special reward like a banner/ flag or something, but if there is not an idea that i think would be funny is if you added a flag/banner that just says "can i have a key" or something of that genre, whatever the most copy pasted form of people asking for keys the last week for when the full game comes out
#pre-order / beta/demo reward idea
small merlin
frigid remnant
This would be funny, but its up to Deady and company if they want to reference running out of keys.
And they should, it was quite the ordeal for them.