I would say this is the ultimate goal, but Foreman supports huge variety of systems including Windows or networking gear and I do not expect those vendors to ever bother implementing this. What you say makes sense for bootable containers context, however, Foreman main provisioning goal is to deploy systems the traditional way: packages, OS images or whatever the OS supports.
So there will always be a Foreman community maintained “3rd party OS” registry I suppose. But you make a great point, Foreman must support any number of “other” registries to cover both official, unofficial and Foreman one.
This also highlights one thing I am missing from my proposal - public key must be published alongside the official repository so PXE files can be verified. In case of RPM os installation tree it can be placed in the “images/” directory.