The whole draft needed a pass at some point for extensibility but a more immediate concern is to play with various things, we need a way of adding header to an object that the relay can read. The LOC draft has a line about needing this, I think Will had some use cases that needed it, and it would help with experimenting and migrating some pre standard implementations to the current version of the draft.

The requirement would be there was a way to define new headers in extensions, there was a way to indicate if the header needs to be understood by relays, reviewers, both, or neither.