Can't help you out, but if that idea ever leads to something let us know.
The effect I'm looking for is basically the same animated shield bubble shape, only instead of being a whole bubble, only the edges of the bubble are visible and the center is completely clear. It would appear that way at close-ish range, but not at long range, but I figure it doesn't matter quite as much what NPC-sized objects look like when they're really far away from you... this would also mean that any shield you have around yourself would be pretty invisible in first-person view.
The way to accomplish this would be to black out the alpha channel on the fullsized texture, but leave the alpha on its MIP maps at normal brightness. I have already accomplished a similar effect using this technique, using it the opposite way (leave the fullsize alone, black out the MIPs) to create a shine effect for a moonstone, by applying the altered texture as a glow map on top of the base moonstone texture. Like the shields, this effect only works at close range, but it does work. Unfortunately I don't have pictures, but I did notice that it made the edges of the model's polygons look sharper, so this would need a really high-poly model to work as intended.