Personally, I would say yes, new namespace. Because I think an XHTML module should be a radical departure from HTML, not in terms of it's output, but in terms of it's construction.
An XHTML module should - by default - be built using the existing XML tools, and shouldn't be outputting 'XML-like code' but actual machine-generated bona-fide XML.
From the otherside, outputting XHTML and using it in a HTML context is almost as bad as outputting non-XML in an XHTML context. <br />, for example, is invalid markup in HTML4.0 (strictly speaking). Of course, browsers will ignore the '/' but you're giving up clean HTML and getting tag soup. Some browsers, especially Moz, might even start going into almost-standards most/quirks mode, or something, which is even worse.
Strictly speaking, I don't think one is a subset of the other, but I would love to see how you propose to do XHTML modules. By thinking about implementation, I'm probably thinking on very different lines to you (or am I?).