The debloat-testing kernel tree

That seems rather unlikely to work in an end-to-end scenario. It also wouldn't respond to transient conditions unless you kept asking over and over. And what about trust issues? Do you trust the information you're getting?

The only thing you can truly rely on is the observed behavior of your own TCP streams--ie. how quickly you get your ACKs back, whether or not you experience traffic loss, and so on.

If you're willing to give up on end-to-end architecture, then yeah, you could coordinate bandwidth-controlled "virtual circuits", but I thought the whole point of IP was to keep as much of the network as state-less as possible with respect to individual connections, letting the endpoints do most of the throttling and letting the network just focus on routing and providing feedback on the current conditions.