Hello,
Since you are using the ASPX development mode, your master page is located as a physical file within your project folder and you can access its code-behind as well. In the code you can call for instance the
CMS.DocumentEngine.TreeProvider.SelectNodes(String, String, String, Boolean, String) with following parameters:
siteName (String)
aliasPath (String)
cultureCode (String)
combineWithDefaultCulture (Boolean)
classNames (String)
As you can see, there is the className parameter where you can specify your document type.
Best regards,
Jan Hermann