Understanding Radiant application layout

Generally speaking, since Radiant effectively runs out of
RAILS_ROOT/vendor/radiant, when, if ever, is it appropriate/correct to
add assets to RAILS_ROOT directly?

Does it ever make sense to install a controller in
RAILS_ROOT/app/controllers, or custom routes in
RAILS_ROOT/config/routes.rb, for example?

If so, when would that make sense?

It doesn’t even appear that Radiant adds these standard locations to the
load path when Rails starts up.

Thanks,
Wes