FWIW - I just migrated my old WordPress site to Heroku, but I decided to keep the WP database structure intact.

So, I made a little Sinatra app with an ActiveRecord adapter that can understand the WP db schema.

It's just a hack to support my immediate needs at this point, but I think the concept is a good one.

http://github.com/trevorturk/trevorturk

Holler if it looks interesting to you.