Brian also pointed out to me that you can leave the
MasterPageFile attribute value as "" and the designer will then
automatically render a test master page that just lists all of the
content-placeholder controls in editable regions. This might allow you to
avoid having to define a test master page for your site.
Here is a simple sample that shows all this working (you can
switch the default.aspx page to use either the submaster.master or
submaster2.master page that is based on mainmaster.master).
Hope this helps and is useful,