Can you go to the manufacturer's website and download the installation instructions? That would be best; otherwise, you'll get generalities.
1/2" durrock is strong enough on a wall to support a tiled wall, I don't see why it wouldn't work under the panel. Now, if you really need it is another story all by itself.
The durrock (or any other brand of cbu) is NOT waterproof, it is just unaffected by moisture. My guess is that the manufacturer will probably request a vapor barrier behind the panels - simple plastic sheeting will work. Now, since the panels are water and moisture proof (except for any failed seams), they may not even require anything.
When using cbu as a tile substrate (which is what it is designed for), you use alkali resistant fiberglass mesh tape and embed it into thinset. It is easiest if you do this while tiling, otherwise you usually end up with speedbumps that make tiling (or putting up a panel) difficult, since the surface ends up with a hump at the seam. The mesh tape and the thinset at the joint is there, not like doing a drywall seam, to create a monolithic surface, not a pretty paintable surface like with drywall. Since you won't have tile and grout to crack, my guess is that if they want cbu up on the walls, you won't have to do anything with the seams. I'd still put up a vapor barrier behind it and bring it down to the lip of the pan. My unprofessional opinion.
The manufacturer's instructions always trump opinions if you want a warranty.
Also note, it may offer setting the pan without a mortar bed as an option...don't take it - use it and you will increase the life and your satisfaction with the install. Make absolutely sure that it is sitting level in both directions, or you will have problems with the walls and draining, too. That is another reason to use mortar under it - you can smush it down into the stuff, it is fully supported, and you can get it level.