Two different animals here
You're quite right, but my point is that as far as the GPS is concerned, an approach is just a sequence of waypoints. Whether those form part of a published approached, or whether they're user-defined ones that you made up as part of your home-brew approach, it neither knows nor cares.
I'm not recommending home-brew approaches, just saying that it isn't too hard to figure the mechanics of how to fly one.