Hi Chris,
I understand exactly what you're asking. I like to take the same approach of accessing a params hash in the rails controllers regardless of client, but send AR models from Rails to Flex. I really don't care for the practice of sending AR instances back and forth between client and server. It seems to get ugly/problematic fast and doesn't really coincide with "the rails way."
I'm currently working on documenting how to accomplish this along with an (functional, but incomplete) ActionScript generator. I don't have time right now to go into to much detail, but feel free to check out the generator plugin at it's temporary svn location of
http://www.treetrunkmedia.com/svn/plugins/rubyamf_quickly
After installing, you can generate ActionScript models based on your AR models... There's a little config that gets added to the rubyamf_config.rb that may explain more.
Generate the models using ./script/generate action_script_models com.yourdomain.package --include-helpers
So say you have a generated Person class and you are using the AS3 generated helpers you can then do something like...
var myPerson:Person = new Person();
myPerson.name = 'Chris';
RubyAMF.call( 'PersonController', 'create', {person : myPerson.toParams()}, myResultHandler, myFaultHandler);
Again... it is still a work in progress, so I offer no guarantees at this point.
Let me know if you have any questions. I'm in a rush right now, but will have time later to answer and provide more details.