Modular website with XML-based backend in Rails?

I've been asked to find the most appropriate framework with which to
implement the following:

A website with
1. Modules, which separate (partially user-configurable) style, from
user settings, from content. A bit like the igoogle modules.
2. ...which can be inserted in various places in a page, depending on
the user's preferences, again think netvibes, igoogle, librarything,
3. ...which can be inserted via some Ajax magic into an external page.
No problem if this is limited to "no login" modules only.
4. Also the whole thing has to have easy interaction with an XML-based
database and web services. No SQL database available.
5. ...and be easy to use & open source, have many developers who like to
work with it.

I was thinking (2) is fairly easy to do with a bit of
javascript/scriptaculous, also (3) can probably be added quite easily
with some limitations.

So this leaves mainly
1: Is there a standard way in rails, or a package, to easily render a
basic look of a module, with user settings and style in a standard way?
Support for skins? Anything that can help here?
4: I know it's easy to make web services in Rails, but can you replace
the entire 'model' with something so different?

If you know any other solutions than Rails that would work here, I'd
also like to hear them. I'm not expecting there is a full solution
available anywhere, but mostly looking for modules etc. which can help
to do part of this. The focus here is on quickly getting something out
and then drawing in other open-source developers for more modules.

Are you possibly looking at a CMS? Have you looked at Ruby-based
BrowserCMS, which has the ability to create Portlets that you can
embed in a page?