Is it possible for a taxpayer to get a copy and dry-run it themselves?
The source code is actually available, written in a language called 'M'. It's been discussed a few times here on HN before: https://news.ycombinator.com/item?id=25208853 (just to link one example).
* https://en.wikipedia.org/wiki/Hercules_(emulator)
Not sure how hard it is to get the system software.
There are other ways to get time on one, including via IBM in an 'online course' of sorts:
* https://mainframenation.com/mainframe/how-to-get-a-mainframe...