I sort of remember trying to get update-manager to work on M11. It's related to software-properties-gtk (or kde), and in the end is due to python-apt not having a template for the distro listed in lsb-release.
README.Templates:
Wow typosChannel Definition
------------------
The .info files allow to specify a set of default channels that is available
in the dialog "add channel". The .info file whose name corresponds to the
LSB release name is used, e.g. 'Ubuntu.info' on a Ubuntu system.
Furthermore all .info files are used to render the channels presented in the
sources list more user friendly.
Tags
----
Suite: the name of the dist used in the repository
MatchSuite: mainly used for cdroms. defaults to Suite
ParentSuite: the channel only appears as a component of the parent suite in
the add dialog
the components/sections of the suite correspond to the ones of
the parent suite. specified components of the suite itself
are ignored
Available: determs the availabilty of the suite in the add dialog.
defaults to False
RepositoryType: does the repository contain binary or source packages
BaseURI: the base URI of the repository
MatchURI: used for identifing mirrors
Description: description of the suite. the translation is done through
gettext at runtime
Component: a component/section of the suite (ignored if ParentSuite is
set)
CompDescription: humand readable description of the component/section
(ignored if ParentSuite is set). the translation is done
through gettext at runtime
ValidMirros: A file that contains a list of mirrors
Basically we need a MX.info file added to a custom rebuild of python-apt, let me get the source files.
Edit: or a package or something that just adds the MX repository templates to the rest in the correct location.
Also, is MX the distro name, and MX14 the release? I think it works better that way for the future releases such as MX15.