Rails template should allow to specify framework parts

Support of modifying environment.rb with rails templates commands is iirc limited to add "config.gem" options with gem(),

But of course you can put any ruby code in your template file to read+change+write the environment.rb file or do what you like.
(or use run() and any console command)

good sources:

http://m.onkey.org/2008/12/4/rails-templates
http://github.com/ffmike/BigOldRailsTemplate

As Roland suggests, code your own environment.rb (or in the case of Rails 3, application.rb) and use that as a template.