I looked. and finally found the option to force call forwarding:
(Available for Calls)
Incoming calls will: (Popup)
Ring your account phone
Ring your phones in order
Ring you phone together
Forward to <previously setup forwarding number>
So, if I can set up an Applescript (if they haven't dropped support for it...) that opens a browswer window on members.sonic.net, logs in, selects "voice", then selects <account phone number", then clicks on the "Incoming calls will:" and select the last entry, select "update", then close the browser window, it will forward.
Another script will select the first entry in the dialog box.
Finally, get a chronjob app that will run the first Applescript in the evening, and the second in the morning.
I'm not sure how easy it will be to navigate around webpages and popups with an Applescript, but that might do it.
Thanks for convincing me to look at every single option in the "voice" menu - its hidden fairly deeply, and NOT in the "Home" tab, "Your Services" section, "Call Settings" tab, "Call Forwarding" section, where you would expect it.