DOCBOOK: Content model of <option> too restrictive

I'm looking to upgrading some documentation sources from DocBook 3.1 to
DocBook 4.1.  The content model of the <option> element has been
restricted, and this breaks a lot of places for me.

For instance, I have

<option>--foo<optional>=<replaceable>BAR</replaceable></optional></option>

This doesn't work anymore, because <optional> is no longer allowed in
<option>.  I figured, maybe <option> was intended to be used only for the
actual option names (e.g., <option>--foo</option>), but then why is
<replaceable> allowed?

What is an alternative way to mark up something like my example?

-- 
Peter Eisentraut