Not really.

Your guess would be golden for direct-sustained devices. N900 uses the battery to drive the internals, the AC is just too noisy. So it has an accurate feel of the battery voltage since it's under load and not disconnected and charging, thus showing the vdrop on the charger.

lshal also shows last full voltage so it's even better than just guessing 4.25 or so. I just wish that the battery would be red and indicating when charging instead of the useless animation.

But that part of the phone is closed IIRC.