Have you considered using blocks instead of dynamic content? XhtmlString supports blocks. Dynamic content is kinda obsolete since blocks were introduced.
As far as it goes for ContentArea support inside a dynamic content plugin, I don't know. But probably no.
Uuups sorry, missread your question. ContentArea should work as dynamic propery, not sure if the on-page-editing works though.
We had that option in ealier version of EpiServer 7 but seems that something is gone in the latest version. I can't drag & drop blocks in my content area.
Is there any way to use a block type as a dynamic property? It seems that there is no support for PropertyContentArea in dynamic properties.