Hi Mark,
Are you seeing anything the mojoportal system log?
I'm not sure if it will work with extensionless urls. One thing you could do is grab the pageid of the page you are using as the starting node and use '~/default.aspx?pageid=X'. This isn't shown anywhere, it is just used to build the menu, so it shouldn't affect SEO or anything like that.
HTH,
Joe D.