On this camera, the battery can only be charged inside the camera. So I cannot charge one battery while using an other. My only alternative is to charge while I film. Can I do that?
Yes, you can.
I first asked the question on a youtube video reviewing the camera. And I got the folowing response:
Yes, it works. You can do anything while charging.
Now, I have bought the camera and tried it myself, and I confirm that Yes, is it possible to charge a panasonic HDC SD900 while filming.