This isn't tied into the rails.vim add-on, but you can add an additional command to do this for you. I believe this would be far easier than trying to modify the rails.vim source.
I actually wrote a blog post about something similar. For your purposes, you'd want to put something like this in your vimrc:
command -nargs=? RcontrollerApi e \path\to\your\project\app\controllers\v1\api\<args>_controller.rb
You could then call it like this:
:RcontrollerApi messages
You wouldn't have the extra rails.vim sweetness that goes along with the native functionality, though (Like typing :Rcontroller
in a view to automatically go to that file).
Here's an idea that may (or may not) work. I don't believe it will allow you to access the aforementioned functionality but it very well may point you in the right direction.
command -nargs=? RcontrollerApi Rcontroller v1/api/<args>_controller.rb