Josh French has been working on loading extensions in RadiantCMS as gems. I walked through the process and pushed out some changes to the Vapor extension to be loaded as a gem.
There's currently no plan to cram this into the official release of the 0.9 version of Radiant, but I'd like to see it happen. I think that extension development and usage is a sticking point for the growth of Radiant and the community of extensions and developers.
Creating the gem was relatively easy using the jeweler and gemcutter gems, and then (after installing the radiant-vapor gem) I required it in my Radiant environment.rb with
Radiant extensions (by default) have an
rake radiant:extensions:vapor:migrate, this one will
rake db:migrate:extensions and will make the necessary changes to the database since the migrations are loaded.