I think adding the demo information to the end of the file (either the dat file and referring to the level number or the extracted level) should be no problem. The other option is to save the demo into a separate file referring to a level pack and number respectively a single level, as the Supaplex demo#.bin does (it uses the level number).
Creating a demo format wouldn't be the problem either, see for example Cheapo (although the function is buggy and you cannot save the recording).
But I doubt that anyone would be going to take the time to implement the demo function into CustLemm, although it would be useful; that's the main problem I think.