Probably a tricker task for cucumber than most frameworks. At the end of the day though I expect that you'll want to have some classes that manage fixtures, perform setup functions, interact with the sut etc and the cucumber regex matches just call into those classes to manipulate the sut and validate it.

So tests here are just cucumber or rspec tests that verify the underlying frameworks that cucumber drives. So for example if you have partial object equivalency methods that verify that only specified fields on two objects match you would write tests to verify they are correctly matching or failing to match two objects as expected before you use that in your cucumber tests.