The m-let service (javax.management.loading.MLet) defines a text file format that describes MBeans to be loaded.  That format looks like XML, but is not quite.  Here's an example:
<MLET
  CODE = com.example.mlets.MyMLet
  CODEBASE = http://codehost/mlets
  ARCHIVE = "MyMLet.jar"
  NAME = MyMLet>
</MLET>

There's no XML header that would declare a namespace and character encoding.  There's no defined syntax for comments.  Tools that can handle XML files can't handle this.

Plainly, now that XML is part of the core Java platform, it would make sense to use a real XML format here.  Rather than trying to allow both the old format and the new with the same API, the proposal would be to deprecate the existing MLet.getMBeansFromURL methods and add new methods that expect an XML file.  The exact schema for this file is yet to be defined but it would lead to files very similar to the example above.