yes, i did - a long time ago. there are two options:

1) open the main page inside of an iframe. We managed to get it working on one of our commerciall sites; but it required tons of recoding the main zen engine; a lot of issues with sessions and post variables.... spent over 3 months to get it working

2) use ajax for opening the main content. Again, this implies quite a big zen code surgery as you will face the same session and other variables links to the ajax generated divs. I used xajax framework for this and had to change over 100 core zen files.

Best

A