In my misspent youth I wrote a login trojan for VAX/VMS. It circumvented the "break" key trick that you are alluding to above. I had to drop down to Bliss-32 and use the $QIO syscall but it can be done. Once I had captured the sysadmin password and logged in once to prove my achievement I never ran it again. Learned many interesting things though. My task was made easier because DEC published the source code for the OS, albeit as Microfiche. I was therefore able to study the terminal handler code and figure out how to make a trojan that perfectly emulated the regular login behavior including all its timeouts and responses to various control keys.

Oh, and the VAX-11/780 I had hacked into crashed due to a memory board fault in the minute after I had logged in with my snagged admin password. I spent the remainder of the weekend sweating that I had broken the VAX since I had no idea what had happened. I had just given myself all 32 of the VMS account privileges when it went down.