Great!! Good job!
I'd like to try it on real hardware, but I don't have a comfortable mass storage device for my MSX (just my SCSI setup, but I can only transfer stuff via floppy and it hasn't been running for over a decade, so it's probably broken). Is it possible to output to two floppy disks instead and later concatenate the files? (This was the advantage of READ-DMK 
)
Or perhaps its possible to use some kind of compression to get the output DMK small enough to fit a single 720kB floppy disk? (This is what Alex Wulms with with D2F, see http://www.msx-plaza.eu/home.php?page=misc/xsaformat - but then we need a way to uncompress it somewhere too.)
I did a test on openMSX, though:
- start openMSX with "Boosted MSX2 EN" and IDE extension.
- start your DMK tool
- put DMK in drive A as disk (The Shrines of Enigma)
- let the DMK tool dump to a DMK file on some IDE partition
- dumping was completed
- after dumping, export the DMK file to my local drive
- try to start the DMK file with openMSX
- expected result: see The Shrines of Enigma boot. Actual result: I see the Sunrise logo, but it hangs after that. (Probably the copy protection kicks in...)
Of course this means there could be either a bug in openMSX or in your tool.