How to implement user editable templates?

In my application, every user has a mini-portal, where his visitors go and I want this to be customized to fit the rest of his website (on his server).

Ideally, this should include HTML files, which users can upload and will be used as template for their pages. The HTML files can include a single element, that I want to replace in my view / controller to pass the page contents that Rails generates. Effectively, this would let me wrap the Rails contents inside a user editable template.

All I need is a single HTML file per user. I'll instruct users to keep images and stylesheets on their server.

What would be a good way for implementing this in Rails?

Thanks, Helzer