There's no way to view other layers, but there is a feature in Liquify which helps quite a lot. If you run Liquify, then right click while over Liquify's canvas, it gives you a list of choices for the background. One of the choices is Clipboard. Before running Liquify, you can make selected PDN layers visible, then use Copy Merged (Ctrl+Shift+C) to copy them to the clipboard. You can then run Liquify and use the copied layers as the background.