Multiview: Format fallback from fbml to html (register_alias/request.format)

I'm using Rails 2.1 with the Multiview template mechanism to create a
hybrid Facebook and public site. For the most part, there are
analogous views across the two. Since Facebook supports a subset of
HTML, some of my HTML views and partials could be shared across the
two.

I have registered a Mime type alias for FBML in my mime_types.rb:
  Mime::Type.register_alias("text/html", :fbml)
and have a filter in my controller to set it appropriately:
  request.format = :fbml
That all seems fine.

I would like to use an FBML template (e.g. "foo.fbml.erb") if it
exists, but otherwise fall back to an HTML template (e.g.
"foo.html.erb"). I have seen references that it should work like this
already (e.g. http://locomotivation.com/2008/06/28/redesign-your-site-in-place-using-rails-custom-mime-types),
but I get "Template is missing" errors if the FBML template is not
there.

It seems to work to name common templates without a format (e.g.
"foo.erb"), which I am doing now, but is there a way to get it to
default back to HTML first?

Regards,

Paul