Use url_for with singleton resources?

Has anyone had any success with url_for and singleton resources?

I have a singleton forum nested under both groups and places:
  map.resources :groups do |group|
      group.resource :forum
  end
  map.resources :places do |group|
      group.resource :forum
  end

So I have paths like /groups/123/forum and /places/456/forum.

In the forum controller, I do what essentially amounts to
  @owner = Group.find_by_id(params[:group_id]) ||
Place.find_by_id(params[:place_id])
so that @owner points to the group or place that has the forum in
question.

In the forum views, I want a link that is polymorphic in relation to
@owner: it should resolve into group_forum_path(@owner.id) or
place_forum_path(@owner.id) depending on the value of @owner.

I've tried url_for([@owner, @owner.forum]), but I get this error: "You
have a nil object when you didn't expect it! The error occurred while
evaluating nil.to_sym". Debugging it a bit, I find that the method
name is correctly resolved into group_forum_path, but this method is
then sent _both_ arguments, where it should only be sent the @owner.

Is this a bug? Am I using url_for incorrectly? Anyone have a solution?

For now, I wrote my own url helper to work around it. I might try to
fix url_for if I find the time, but first I want to find out if it is
in fact broken.

So if I patch like http://pastie.textmate.org/86956, I can do e.g.
  polymorphic_url([@parent, @forum], :singleton => true, :routing_type
=> :path)
In practice I'd wrap it.

Is there some way to figure out if the generated route is a singleton,
though, so you don't have to specify it manually like this? I found
http://dev.rubyonrails.org/ticket/8830 /
http://dev.rubyonrails.org/attachment/ticket/8830/polymorphic_routes_for_singleton_resources.diff
which makes use of "plural_route", but a grep in edge doesn't turn up
anything of the sort. Google only finds the diff in question.