All the binary exe's are wrappers that load the python source code
HERE.
Looking around, the only effort made to support KEPUB is telling calibre in a few key places to treat a KEPUB as an EPUB (for
very basic functionality).
As far as send-to-device (and most of calibre) knows, KEPUB is Yet Another Unknown Format.
All that is needed I would guess is to reimplement it as an ebook type plugin that subclasses EPUB, thus making calibre intrinsically aware of KEPUBs.
And once that is done, it can (and should) incorporate the various tweaks and refinements that the current user-installed Output plugin does, etc. -- then calibre will have builtin support for KEPUB.