So if I built my own personal ad network, I could make a PR and have it be accepted?
An external contributor is currently building first class support for "My simple ad network that just wants to put an image with a link on the page" https://github.com/ampproject/amphtml/issues/5541
There are other ways to do it, though.
I think with the numbers of ad tech companies coming on board every week one can safely say that the process is working for them.