Could you temporarily turn off all the steps in the action after the Unsharp Mask step, then run the action on a photo. We need to see if the white borders are there before the action places the watermark.
Just to confirm, you turned off the checkbox in the action?
http://www.damiensymonds.net/images/actions/wmact17.gif
In that case, did you crop at all, at any time? Manually, I mean?
Yeah, we can do this. Obviously the key to it is taking a bright photo in the first place. See how this photo has a white wall, pale skin, blonde hair, white clothing.
Yes, I'd say it's possible.
But I need you to tell me more about your vision. You say a white background, but how would you intend to blend that with the ground?