It can be done with relative ease. There is a commercial tool somewhere that does it, because I've run across many PDF files that use a DjVu like structure for scanned books.
It won't be perfectly lossless, because the IW44 compression of the color layer will need to be recompressed as JPEG or JPEG2000. The JB2 mask layer can be losslessly recompressed as JBIG2 or CCITT G4 Fax.
It won't be perfectly lossless, because the IW44 compression of the color layer will need to be recompressed as JPEG or JPEG2000. The JB2 mask layer can be losslessly recompressed as JBIG2 or CCITT G4 Fax.