The mini disk letters were your command search order, like unix path. But internally it was a linked list so you could rearrange the order it you wanted to.

File mode 0 files were supposedly private, you couldn't see them if you accessed the disk read only. But the access command ran from the transient area, so you could load it, no-op the test for char '0', and then execute it to access the file mode 0 files on a read only disk.

Mini disks supported sparse files. So if operations didn't migrate mini disks correctly, you could put a 100 MB file on your 5 MB mini disk (this was the 70's), that would stop them.

There was a diagnose command to read your 3270 terminal screen including the field attribute chars which you could replace with markup tags so you get screenshots for your documentation.

There was a lot of hacking going on in IBM at the time. Just wasn't public. If Peter Capek's VMNewsletters and some of the internal discussion groups were ever published, you would know.