Austin, Thanks to the start you gave me, I've managed to get this working today:

Here's the code from my model that generates a PDF from a Prawnto view and saves it to a file:

av = ActionView::Base.new(Rails::Configuration.new.view_path)
av.extend ApplicationController.master_helper_module
av.controller = ActionController::Base.new
av.controller.request = ActionController::Request.new({ "SERVER_PROTOCOL" => "", "REQUEST_URI" => "", "action_controller.request.request_parameters" => {:format => :pdf} })
av.controller.response = ActionController::Response.new
av.controller.headers = Rack::Utils::HeaderHash.new
av.controller.prawnto :prawn => { :page_size      => 'A4' } # your Prawto settings here....
pdf = Prawn::Document.new(av.controller.instance_variable_get(:@prawnto_options)[:prawn])
pdf_body = av.render(:file => 'reports/render_me.pdf.prawn', :locals => {:pdf => pdf}, :layout => false )
File.open("MGPtest", 'w') {|f| f.write(pdf_body) }

This is a first draft of a working version, so can probably be cleaned up a litte (or lot!).

Of course, YMMV, and this is a total cludge of the MVC principles - but sometimes you've just got to...

Hope this helps out there.