Plugins for admin side of Radiant?

I’m going to need to make some changes to the admin side of Radiant
(add a few controls, an extra model or two, etc.) for a few upcoming
projects, and I’d like to extrapolate them into a plugin-type system
so that it’ll be easier for me in the future and because I think that
others might get some use out of them.

Is it possible to use plugins for the admin interface, and if so, does
anyone have a good tutorial on writing plugins for Radiant?