Route directly how? Write my own HTTP layer? It's so easy to take advantage of Rails' routing and view rendering.
Well, instead of fighting rails, you could just use the lower level stuff it is based on which does the bits you want:
Routing - https://github.com/rack/rack
Templates - http://www.ruby-doc.org/stdlib-2.1.1/libdoc/erb/rdoc/ERB.htm...
Rails adds quite a lot of overhead and complexity, not to mention memory usage, so if you're going to bypass most of it, it might be better to start with something more bare-bones?
The point here is not to look at the devise/router serving http but the porting of the data & business logic to an adaptable, multi-purposed api that is not tied down to the infrastructure in any way.
I've used this approach within a Sinatra driven app, CTO said i need to use Rails now for some other thing, a few hours to make the switch, not rewrite the whole thing.