I think the challenge here (although I could be wrong) is that Custom Business Days get treated the same as Business Hours as far as the 'Calendar' step in the script is concerned, so they can't be differentiated by the script. I agree that it would be a useful feature!
I'm assuming that you are using Finesse, so a possible workaround is to grant your supervisors with Application Management capability in the Supervisor Capability View settings on UCCX. This would allow them to change the outgoing message as and when required from their Finesse client. It is clearly not as convenient as being able to script it but as I said is a potential workaround.