The restriction is that you call <master> or at least <master src=/www/blank-master> in your page.
You also need to define rows/cols which wasn't obvious to me.
There is a parameter UseHtmlAreaForRichtextP in acs-templating that will turn all richtext-textarea's into htmlarea's by default.
The "B I U links" of the richtext widget remain visible AFAIK.
I guess it is not save to allow users access to all functions that are offered by htmlarea (hyperlinks/IMG/HTMLsource), or is openACS' check for valid tags/attributes considered sufficient?