Load the layer transparency as a selection. Enter Quick Mask mode. Image > Adjustments > Threshold [set to zero]. Exit Quick Mask.
Sorry, Semaphoric, I'm not sure if you didn't understand me, or I'm not understanding you. I want to select all the transparent pixels in a document (multiple layers in a document), not a layer.
Wasn't at the computer before, so I couldn't check, but this seems to work (you can make a Action of it):
- With all layers visible, Stamp Visible (Control+Alt+Shift+E)
- Select Transparency of the stamped layer
- Enter Quick Mask
- Threshold = 255
- Exit Quick Mask
- [Optional] Delete the new layer.
You could change the location of the Invert step, but you may have to change to Invert Selection and/or Threshold = 1.
I tried exactly that, but I misunderstood something and thought it would not work. It does. Thank you!