It's just a matter of a custom texture file. Make your new overlay with transparency ( in GIMP for instance, but could be pretty much anything ), save it in PNG format and paste it into jars/bin/minecraft.jar/... Then in my paste change this part:
%blur%/misc/pumpkinblur.png
... into:
%blur%/<your path to the new texture inside the minecraft.jar>
That's what I was rambling all about all this time This method has one big disadvantage: you will have all of these HUD elements like: experience bar, handy inventory at the bottom of the screen, health and hunger levels covered by the overlay texture. There's no way around it unfortunately. Unless you modify the Minecraft. Or a miracle happens and the Forge team puts some render handlers in there to have some control over layering the overlays but I don't think it's gonna happen any time soon
Cheers