There is a way to extend the format. As it has version number. It does have some metadata fields (drive geometry, compression formats, version of mame created

There is a way to extend the format. As it has version number. It does have some metadata fields (drive geometry, compression formats, version of mame created with, etc). The trick would be getting the MAME team to accept the changes. Just dumping it on the end of the file I would guess they would not be too happy with. There are a number of requests out there to extend the chd format and fix a few things. They are currently tracking some of this info in XML files (called hash files). They would be down with more accurate information though. So you would need a proposal that adds more accurate information and gives them something to work with. More like getting all the info that something like the redump project tracks in there would make them very happy. There is a separate project that some other emus use libchdr which is a soft fork of MAME. I think they are trying to track closely to what the MAME group is doing but let other emus use it.