Sidenote: A quick, mostly backwards compatible, change could probably add cwd to the existing per-process tuple in self.processes as an extra item, couldn't it?
No. Most usage of this tuple is unpacking, which makes it really really not backwards compatible.
> 4. Decide we're going with "incompatible change", and move processes to a private attribute.
I see no reason for accessing those attributes from "outside of ProcessMonitor" but if Twisted claims not to break existing code without previous deprecation warning, such an approach would break that promise (full policy details here http://twistedmatrix.com/documents/current/core/development/policy/compatibility-policy.html).