Ok, adding custom icons in a plugin should now be doable as of
https://github.com/discourse/discourse/commit/47cbfb14989bcf828c66b211827cb9355214e71a
So, basically, add the SVG sprites to your plugin, in plugins/svg-icons. See https://raw.githubusercontent.com/discourse/discourse/master/vendor/assets/svg-icons/discourse-additional.svg for an example. Next, register your icon, and you should be good to go.
(I would prefix the icon ids in the plugin, to avoid any potential clash with FontAwesome icon names.)