Honestly, I don't care very much about enforcing one-liner syntax in rspec-core's cukes because we generally don't use it in rspec-core's cukes except for the ones that are demonstrating the one-liner syntax (where we need to allow both forms anywhere).

rspec-expectations uses the one-liner syntax extensively because it helps us show many usages of each matcher without much extra noise of surrounding code, so we wanted to enforce it there.

But given that you've already set this up, this is great :).