If one implements an XModule/XBlock mixin currently, there's no way to override the XBlock XML serialization methods (parse_xml/add_xml_to_node) in order to perform custom XML serialization. This is due to non-existent super() calls, the method resolution order (MRO), and the lack of method symmetry.
Here's what needs to be fixed:
Resolve the MRO so that super() calls to parse_xml/add_xml_to_node from newly implemented mixins will call the base serialization methods for both XModules/XBlocks.
Make the methods symmetric by constructing a block first outside the method - and then calling the appropriate method to add attributes/parse out attributes. Instead of returning a constructed block, the code will operate on an already-constructed block.