Sandstorm had a radically more innovative model of hosting: instead of helping you host apps, each document was its own instance. This was really interesting, but as this article notes, did need deep integration with the apps.

I'd be interested to see if your more pragmatic approach plays out. Being able to start by taking apps "off the shelf" largely unmodified, then gradually adding integration via standard proxy features, seems sensible.

In any case, doing single sign on between apps is just a very sensible and worthwhile place to start.