To change the default master on dotfolio creation, you might want to take a look at /dotfolio/lib/install.xml
This install.xml file is called or used when creating the subsite of a new folio. I think it's possible to set the defaultmaster parameter for the new subsite there.
TO determine the subsite url, see if the proc "dotfolio::user::dotfolio_url" is what you are looking for.
