I'll put this in Feature Requests, as I think FD could use this functionality, but thought I'd also mention how we did it in the existing product.
Basically we used the Product Portal function (intended for when you want unique experience/branding on a particular product). In this case I made the "product" = "partners" - in this case sales partners so we only wanted them to see sales content (using Articles). Where we needed forms, we used embed tags in Articles combined with FormSite (surveymonkey, etc would also work) for things like Deal Registration.
We point Partner SE's (the propeller-heads) to our Support portal - the neat thing is we get a common login across both, and can share content across both portals as well)
Main effort was hacking up the HTML to remove most of the functionality - basically it's just displaying Articles. We may turn Forums on at some point - or even ticketing.
(good stuff is behind login so attached a screenshot - not terribly pretty - need to get our marketing folk on it..)