Resize an image in a single layer in photoshop

The simplest solution would be to use a mask, but that would require you to remember to make the mask just before you export.

If you want to go this route, select layer 3, and then ctrl for windows/or command for mac click the layer you want to be the basis for the transparency, and then click on the icon down the bottom of the layers panel with the circle in a square. This will give you the transparency you want, but it's entirely manual and you have to remember to update it when you edit the black layer.

You could create an action that selects the layer you want to use and makes a mask from that, but you'd still have to remember to run the action...

CS5 Extended has multi-layer fill. If you create a texture with your tile (probably want to run a 50% offset filter, too) you should be good to go. I don't know if you are running this revision, but it might help. If you're running an older version, I believe gIMP also has multi-layer fill for textures, you should be able to perform this step in gIMP and then pop it back into PS without any deprecation.