One of the things that Acorn 2 added was the ability for developers to add their own image types to Acorn. So "The Stig" has written a plugin for Acorn which opens up Painter RIFF files (including the layers). Since RIFF isn't an open format, it uses Painter's frameworks to get this done (so you'll need Corel Painter or Sketch Pad installed for it to work).
You can check out ACRIFFLoader on GitHub. (I haven't tested it out myself though).
Someone needs to write a multi-layer TIFF IO plugin next.