OK, I got one step closer. I used another GMail account to create another GV number and configured SP2 with that GV account on my Obi212. I then put SP2(1234567890) as the forward number for LINE physical port. Where 1234567890 is the GV number that is active on SP1.
So when I call my POTS number, it goes into LINE port on the OBI, then uses SP2 to forward that call to GV number 123-456-7890, which GV then forwards call to SP1 on my Obi212.
It works, but unfortunately, the Caller ID does not carry though. The displayed caller ID is the GV number for the SP2. Not having a correct caller ID is a deal killer however. So I guess there is no way to do this.