Here's how I think I'll solve it for a couple smaller clients:
Create a dummy extension that goes straight to voiecemail or a Digital Receptionist if the call needs to move on somewhere else (like another DR) after the weather message is played.
The voicemail greeting or the Digital Receptionist audio message will be pre-recorded.
In addition to the default "inbound rule" we created, I'll create another inbound rule that forwards all calls to the dummy extension or Digital Receptionist we created above.
I'll instruct the customer to login to 3CX, open the "Inbound Rules" window, and simply 'move up' the Weather Rule to position #1. Then once the offices re-open, they'll move it back down to the bottom, below the default inbound rule, effectively disabling it.
The only gotchya is that the default inbound rule routes calls based off the DID (in our case a * wildcard since the customer only has 1 DID). You can't create another rule with a wildcard DID, so you must create the Weather rule using a wildcard CALLER ID.