I have abandoned the update as I have been
advised that a new architecture is coming which will handle authentication
via plug-ins and as such the core developers would rather not add functionality
to this area.
I would suggest you get involved in shaping how that works with plugins. I know it was a planned feature back when I was still working on Mantis and the plugin system, but afaik there's no one else left doing work on the plugin system, so you could have a lot of impact in that area if you are interested in it.