I think it's be a bit of advertising for Bitcoin for people that walk past and people may buy as novelty gift etc. (if done as fancy paper wallet).
I'm considering trying this in London, what do you guys think?
Not practical because all those that will buy or sell to you at the end of the day will need a computer to load them into the blockchain somehow and the bad people can come up with fake paper keys that they will be selling that can create a bad name for Bitcoin.