The best way to integrate it is not to actually integrate Mephisto and your app at a code level, but run them as two separate apps and just make them act like they're part of one site. The user can't tell the difference unless they look very closely.
For example, take a look at www.BuildingWebApps.com. Click on the Blog tab, and you'll see the blog, and the URL will be blog.BuildingWebApps.com. The blog is running Mephisto as an entirely separate application; it has just been styled to look like the main app. The DNS directs blog.BuildingWebApps.com to the server running Mephisto, while www.BuildingWebApps.com goes to the main app server.
P.S. There is a separate group for Mephisto that you may find to be a better place for this discussion.