It won't work for Linux users, unless the limitations of MakeBook are acceptable (or can Wine be used with DropBook?), but a simple work around for eReader output is to write out PML and let
DropBook do this rest. This is the approach taken in
ereader2ereader in two steps. There are other advantages of doing this, such as simplifying debugging of conversion issues (experts on PML can see the document source). Also, what Calibre's eReader output should look like is either identically PML -> DropBook or that plus some extra frills. So developing direct eReader output will be simplified if the PML option is in place.