Very cool, love the layout generator! There's also http://buttons.cm/ for non-image buttons and http://backgrounds.cm/ for background images, both are cross-client compatible (even with our friend Outlook!).
I'd suggest solving this by wrapping the inner <A ...></A> in condition comments like: <![if !mso]><A ...></A><![endif]>
I've been meaning to write up a blog post on this...