I was thinking that people could upload a payment protocol file somewhere once (like to their personal web page, or shared via dropbox or google drive or some custom new pastebin style service), and then just encode a regular bitcoin URI into the qrcode on the billboard.

Likewise, I could attach a payment request to an email and send it to you, and now you can pay me whenever you want forever.

Getting a little static piece of data to someone once should be something we can make easy. Constantly refreshing it, on the other hand ...