Here's what I do.

1) sip (G5) setup on N900
2) GV forwards to G5

I am using TOR, but DialCentral should work as well. Either should be able to use GV's callback feature, with G5 as the callback number.

So, TOR monitors GV for SMS and voicemail. SIP monitors G5 for calls.

Scenario 1: Someone calls GV number. GV forwards to G5, N900 rings SIP call. I don't use 1 to accept, I think there is a bug for using the dialpad with SIP, so you may have to sacrifice this feature.

Scenario 2: I want to call someone. Using TOR, I can select a GV contact or dial via dialpad and select GV as call type. A call is initiated, hangs up, then GV callback to specified number (G5) occurs. Since G5 is the callback number, call comes in via SIP. Anwer, then GV dials the other number and completes my call. Dial Central can do this as well.

DialCental and TOR are similar in features, but TOR is more integrated into the N900. SMS come through Conversations, and GV contacts can be called like other contacts.

The trick is to set your callback number to your G5 number and you should be all set.