We've had Duncan out with a harness before, but it never really seem to work well. We're in a quie-ish - but built-up area, and we're not happy about them being out at all. Maybe if we moved to a quieter, more open-spaced area - then we'd review that, but definately not where we are right now.
In our old place, Leo would be in and out like a yo-yo. There wasn't a catflap for him to use, but instead Deborah tied a string of bells around the inside doorhandle. Leo worked out pretty swiftly that if he jangled the bells, we'd come and let him out. Which we did. Often!
No system for him to come back in, as it was a shared vestibule - so we put a catflap on the shed and made a comfy hammock in there for him. So, when he can't get into the house, he'd go there. At teatime and at night, I'd just call for him and in the darkness you'd hear the catflap flapping and Leo meaowing all the way to the door, exciting to get back in! Brilliant stuff!
Still miss that guy.