^ Yes layers exist everywhere, but where to start?

To achieve layers, the lowest layer is drawn first, followed by more higher up layers.  Some layers, like having a custom background may be skipped, as you can't have transparency/alpha with custom backgrounds.
Also, I don't know how it's internally stored.  It would be either:
1) Just loaded and drawn in a specific order
2) PSP stores each layer with a corresponding z-index, after all layers are loaded, it's drawn bottom-up

If the latter case is true, a simple hex edit may do the trick.  But if the former, it's unlikely a simple hex job would do the trick.