We'll here's where I'm thinking right now. First, you get your email list going. I'd say everyone gets a monthly newsletter, its a good balance between visibility and annoyance. Then, as part of your regular newsletter, pubs that are silver rated or higher get to be part of the newsletter. They can say what new beers they have, what events are going on, etc. I know its advertising, but having that group of contacts is the value that ICB can provide.... but it will only provide it if they cater to the needs of ICB members.
Newsletter layout:
1) Beer interest article
2) Homebrewing article
3) New things happening at ICB
4) New beers on tap - County Specific
5) Events at member pubs - County Specific
Those 3rd and 4th sections are gold for a pub because they cater to a dedicated interest group... but only if you meet them halfway. They'll likely even pay for the privilege.
The neighborhood local may be an impenetrable bastion of mediocre beer, but you're more after the 10-15 places in any county looking to differentiate themselves.
Likewise, and this is less [edit] of a runner, you allow some members of ICB to be the compliance police. Anyone who pays dues (say 15 euro a year) has the right to notify ICB if one of the pubs isn't living up to the deal. If a pub gets enough complaints, they lose their cert.
Its all hypothetical, but that is a grass roots structure for you. Pubs will pay to be part of networks that can guarantee them customers, and by increasing their visbility with a specific group, they increase that chance.