Thanks to Aslam's fantastic blog post on the topic, I decided to try and get the docs via YARD.
First install YARD
gem install yard
The obvious step is to fork the rails source and simply perform yardoc which is an alias to doing yard doc. This will output the nicely YARD-formatted Rails API into the /doc folder.
If you wish to view, say the Rails API via your own local YARD server, simply perform
yard -n --single-db && yard server
Point your browser to http://localhost:8808 to view your Rails API documentation, all nicely YARD-formatted.