There's a pretty big difference in size between the instinct battery and the Moment. After reading the post about using the instinct battery I looked at the 3 extras I have on my desk and slid one in and there's no way in hell I'd try using it. I'm sure you could get the phone to light with it but the battery is not only smaller, the contacts don't line up the same difference from the edge as the m900 battery does. It seems to me ther would be a chance of the battery sliding far enough in one direction to possibly cause it to short out. I don't know about you but if I fry this phone I'm up the creek.
I do however see a possibility of modding the instinct battery charger to fit the bigger battery but it would involve a dremel, some epoxy and some hard plastic... I bet it could be dome pretty easily but it would be straight ghetto...
Here's a pic of the two batteries side by side...