Basically the invasions are just hack'n'slash quests, where a lot of relatively easy mobs and a few 'boss mobs' load in a limited time and place.
But of course even that could be turned into a RP quest, if someone actually bothered to initiate the roleplay, when calling to arms and handing out defence tasks for everyone.
And of course if they did, they could always post a log of the action on the RP boards, and get awarded for it.
It's all up to the players, the mud becomes what they make it to.