Radiant loads plugins just like any typical Rails application.
If you want to load custom routes, they are loaded from the plugin's config/routes.rb file. Custom controllers and views? They are pulled from the plugin's app directory.
So if you need to pull in an application plugin which does all this for you, you're free to do so. There's still plenty of work to be done with the core to make things simpler, but loading plugins like a regular Rails application is no problem. You're still constrained by the code in ApplicationController if you inherit from that, but you don't necessarily need to inherit from ApplicationController for any particular reason.
But Radiant's extensions can also bring plugins with them. So, for example, if you want to work with the AdminUI in Radiant but want to pull in a plugin's code there, you can work with vendor/extensions/your_extension/vendor/plugins/your_plugin. Yes, that works too, even the config/routes.rb
In both your project's vendor/plugins and your extension's vendor/plugins you can load the interface you need
If you're working with Radiant, you might be able to use some development direction or even a simple consultation to give your plans a head-check. Give me a call