I handled this with what I called a pre-AA greeting, a greeting that is always present but normally blank. The CRA recording list was <pre-AA> <1 second pause> <normal AA> <3 second pause>.
For normal operations, there is a 1 second pause before the auto attendant greeting is played. Not noticeable by anyone.
For special occasions, record pre-AA greeting. You hear the pre-AA greeting, 1 second pause, then the normal AA greeting. When the special occasion is over, record a blank greeting (hit # twice quickly).
The beauty is simplicity. Call into the admin mailbox from anywhere to record the greeting.
Alternatively, pre-record several greetings then call into the admin mailbox to swap them into the CRA. We did that for schools, with pre-recorded greetings for delayed opening of various durations.