Last weekend I built a prototype of a service with about a dozen objects and as many associations. I was able to cover a good size surface area quickly using nifty:scaffold in Rails.
This weekend (as soon as I pull myself out of bed today) I'm going to (as Sam pointed out) use Grape to create the versioned API facade in front of it hoping that Rack lets me connect to the Rails all as easily as promised.
My goal is to have a robust mock that supports reads n writes in just 20 or so boy-hours of work.
The final step will be setting up the OAuth provider.
I'll write something up about the experience once I get thru it all.