We are limited to 4 channel images so you wouldn't be able to get r,g,b,a,depth into a single texture. In general though there's nothing to stop you writinga pixel bender 2D filter that then gets used as input to later stages.
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I have also discovered I can do this with a pixel bender 3d fragment shader, and I think this is the correct way to do this. Thanks!
I'm also interested in processing bitmap data with the gpu. Would you mind sharing your solution?