[electroly]

This process seems to intentionally cut off upstream from having input into the design of the features during development when there's still a chance to change it. What happens when you implement something that you like, release it internally, but then it turns out upstream doesn't like it or wants it a different way? Will you maintain your feature on your fork forever (thus JaneStML), or will you abandon your feature? Is the thought that, honestly, this will never happen given Jane Street's track record in the community and the level of battle testing that this process brings to the design process?

[nequo]

Another possibility is to change the design based on upstream feedback and adjust the internal codebase to it.

By parent’s description, those codebase adjustments are done during the iterative process of feature development too. Upstreaming would just be the last iteration.