This request was made by Kim Marshall via email...
I am fairly sure I have seen it before, but was unable to find it.
This option would show on the Master Password Prompt window, and would completely disable the auto-populate function for all accounts (Default and Custom), but only for this browsing session.
I think this should be a global preference that is disabled by default. When disabled, the checkbox option to 'Disable Auto-Populate for this Session' doesn't show on the MPW prompt, when enabled, it does.