You have to trust the billboard owner too. If you're relying on a third party to relay a payment instruction, that will always be an issue, hence the signing.

Signing a payment request for an individual is easy, anyway, depending on the kind of ID you want. If you want to sign with an email address, just go here with a browser like Chrome/Safari/IE that uses the system keystore:

   http://www.comodo.com/home/email-security/free-email-certificate.php

They'll send you an email, you click the link to verify, and a cert will be generated and installed by your web browser. It's actually easier than signing up for a website. There are lots of other places that do it for free too, I just picked the first one from a google search for [free email certificate].

Once you've got that in your keystore, a wallet app can quite easily be told to sign payment requests with your email address.

For a billboard I guess you'd probably be an organisation or company instead, though an email address can work there too as long as you have a well known domain name.