This is also what a secure attention key is for. Sadly the well known implementation (Windows NT) made it sufficiently obnoxious that it went away.
I can imagine keyboards having a special “password” key and trying to train people that all passwords start with the password key. I don’t know if this would work, but it can’t be worse than Ctrl-alt-delete.
[1] https://en.wikipedia.org/wiki/EROS_(microkernel)
EROS used capabilities to enforce such rules. If you lack a capability to be a system window, you can't pretend to be one.