> use Merb::String
>
> class Foo
> def bar(str)
> str.to_s
> end
> end

Just out of curiosity: Wouldn't the problem with your examples be that
the String class is hard-wired in the ruby parser -- see eg
http://groups.google.com/group/comp.lang.ruby/browse_frm/thread/2a0ad2d4a78f499d/

I'm not sure though if this is still true for ruby 1.9.