small update to say that, if you want to test these with latest svn, then you have to use the following syntax
mess nes -cart datach -cart2 gamename
where gamename can be either a Datach softlist shortname, or the full path to the iNES files for the games (notice that you can also load an headerless 256k dump of your Datach minicarts, so you don't need to add an header if you dumped your own cart)
this is because the Datach Joint ROM System base unit has been properly implemented as a passthrough cart with a separate cartslot for its minicarts