Obviously a guild like this would need some number of active members, if just to set up a kind of headquarter. And those people would then be able to take up arms if neccessary. From what i understand Claims will be protected and your property inside will only be theratened in actual guild wars. Ergo a neutral guild dedicated to diplomacy, assuming it is well liked throughout the lands, might well be supported in case of an attack.
In the end it all comes down to if people are willing to do this level of RP. I think that you would have to wait until the dust has settled after release and most bandits and brigands have become bored. Then if enough of a playerbase remains, we might see this plan work.
As for the external site - it's a step in the right direction. It could for example be estabished that every major guild should send in a weekly report of ongoing wars, diplomatic relations, member count, whatever. Then somone forms it into a 'state of the continent' newspaper. But what you lose completely by this approach is what defined diplomacy in these days. Information was not available everywhere, and sometimes it took ages to aquire. Informants might even spread false information and there was little security that a diplomat sent by one king to another actually delivered the message whithout alteration. Those are feats which a diplomat order might offer to the guilds and get some pay for.
Your site is great btw, i just can't read it
I am German, give us a Russian with decent English and we can do it.