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?