Players can build camps for their armies, etc.. These can have varying levels of fortification.
Perhaps the Devs could consider implimenting the ability to build camps on a trade route, forcing other players to pay to pass or fight. [similar to how players can bribe off potential attackers]
However, the idea of a guild outpost sounds more similar to the idea of a guild city, which, although that idea has been discussed very thoroughly, I am not sure if a definitive conclusion was reached on the issue.
"Yes... I am writing a book. So far I have the page numbers." - unknown author.