I got everything working. What I did:
1. In Obi expert config, there are two star code profiles, A & B. By default, Code7 and Code8 (*72 and *73 respectively) operate on all SP's simultaneously. I edited profile A so Code7 and Code8 operate only on SP1, then edited profile B so Code7 and Code8 operate only on SP2.
2. In Obi expert config, there are two physical phone interfaces, Phone 1 and Phone 2. My cordless phone is SP1 and Phone 1, my fax machine is SP2 and Phone 2. By default, both phone interfaces specify StarCodeProfile A under Calling Features. I edited Phone 2 to specify StarCodeProfile B.
That's it - now I can pick up the handset on my fax machine, dial *72[CC fax DID]#, and all incoming calls to my legacy fax number (now GV) are forwarded to my CC fax DID.
Ianobi: My solution doesn't have any provision for SP3/SP4. I'll try your trunk format if I ever have a need for SP3/SP4.
Azrobert: I have multiple CC DIDs but want to call forward only one specific DID.