What is the use case? Do you want to toggle it from the phone, or change it remotely?
Unconditionally forward? Like I call your extension number and a VM module picks up instead?
Or your extension covers to voicemail, and instead of your mailbox a module answers?
I have an interactive VM Pro module for changing a user's twinning that could easily be adapter to change forwarding instead.
You could create hunt groups that are used ONLY for the purpose of reading their state, and within VM Pro route calls based on that state. Give the user a Hunt Group Night Service button to toggle the routing.