This is an interesting solution.

So the actual app could be at my main marketing domain - perhaps just simply have the marketing site appear when they are not logged in. Once they are logged in, they are shown their home screen.

But how do I handle the file organization for each user. Give them their own subdomain or just do folders? So all the images for marcamillion would be mydomain.com/marcamillion/images, and my home site would be mydomain.com/marcamillion.