I am tinkering on a idea to update addons from a addon manager in blender, the idea was to be able to add git repoisitories and then search and install/update them.
It certainly is possible to do this and it does s not require every developer to implement a updater to his addon. Of course it will only work if the addons are available on a open repoisitory.
If anyone wants to follow, or give some input and later try it or even help develop the addon or use the idea to do so yourself you can find what i did so far over here: