I'll take this to the Synapse team to discuss further, but we could probably easily arrange for one of our CI runs to install Twisted trunk from git instead of pypi, which might be a good start.

This is specifically the approach I'd really rather not take :) and here's why:

1. You want to provide stability for you contributors so that if a problem is introduced, you don't halt development on that unrelated feature to diagnose the upstream issue.
   
   
2. You want to ensure that when users install your software, it works with everything that's currently released.

I'm not sure I follow this. We'd still have CI runs that test against *release* Twisted; I'm just proposing that we would *also* test against development Twisted.

Your point about not stopping development when there's a problem is well noted though. A CI pipeline that runs on a timer might work fine. I'll discuss with the team.

but we have had enough difficulty keeping our CI configuration current based on what cloud provider is falling over this month ;-).

Yes, CI seems to be universally one of those things that is conceptually simple but somehow takes hours and hours to maintain.