Properly locking down a machine just heavily restricts what you can do with it, to the point that normal things that you or I do day-to-day on our own PCs become impossible. Every time you hit a restriction its very frustrating.
I am drawing from my experience as a developer, so it might be worse for me, but I've also heard accountants in the office complaining of similar gripes with their locked down windows systems.
Only the end-users would have rights to the source under GPL, and its unlikely that someone is going to risk their job by releasing the code.
I'm not sure how FOI would work, but I dont think they just automatically get approved.
I still expect it to be done in the open, one of the things Munich got right was upstreaming all their changes, which meant that even when it was cancelled, nothing was lost. Maintaining out of tree changes is just way to much work