How would I go about letting an end-user customise a JS/CSS package using a web from, serving the package when it has been compiled in the background?

I have been looking at integrating a system similar to the Twitter Bootstrap/Zurb Foundation websites custom download section.

Upon reviewing the source in-browser for said sites, it soon became clear that I would need to integrate some sort of Grunt task. I have searched for how to do this, but due to the search terms I am using, it is throwing up non-relative results, leading me to the basics of Grunt, which I am now familiar with setting up. The only problem I have is linking a web form with the Grunt process and having it serve the files to the user.

Ideally I would like a full Ruby on Rails solution, but I don't mind PHP as I am able to integrate PHP with my Rails app.