Hi pikach,
I am not aware of the possibility of doing exactly what you described. For now, the only option I know is to simply enable the built-in Auto Attendant system. This will provide the caller with a three-option menu. Choosing the first option (by pressing 1) will connect the caller to you (until someone picks up).
To route all incoming calls from GV to the Obi's AA, you can try the following. I'm using SP1 in this example, but you can use whichever SP Service your Google Voice is on. You will also probably want to enable a PIN for security purposes if using the AA. This should route all incoming calls on SP1 to the AA:
Voice Services -> SP1 Service -> X_InboundCallRoute -> aa
Voice Services -> Auto Attendant -> UsePIN -> Enable
To modify the time period after the ring before the AA answers, you can re-configure the following in milliseconds:
Voice Services -> Auto Attendant -> AnswerDelay ->
Your original idea is really a good one. Unless someone else knows a way to implement it with current OBI options, it would make for a great submission in the Feature Request sub-forum, as it would eliminate google voicemail very nicely!
pc44