This function works great and was just what I needed for keeping the memory down in my menus, but I would prefer to be able to still encrypt my images through a .wrs file as normal.
No, WRS resources are specific to Gamestudio, and are not recognized by DirectX. You need to unpack a file before DirectX file functions can access it.
For loading a bitmap from the WRS, just remove the old bmap and then call bmap_create.