[jgoguen]

It doesn't seem to matter if I have a configuration widget or not. I have been able to get the conversion to succeed with this code:

# vim:fileencoding=UTF-8:ts=4:sw=4:sta:et:sts=4:ai

__license__ = 'GPL v3'
__copyright__ = '2013, Joel Goguen <jgoguen@jgoguen.ca>'
__docformat__ = 'markdown en'

from calibre.customize.conversion import OptionRecommendation
from calibre.ebooks.conversion.plugins.epub_output import EPUBOutput

class KEPubOutput(EPUBOutput):
    name = 'KePub Output'
    author = 'Joel Goguen'
    file_type = 'kepub'

    def __init__(self, *args):
        self.options.add(
            OptionRecommendation(name='opt_kepub_enable_extended_features', recommended_value=True, level=OptionRecommendation.HIGH,
            help='Choose whether to enable extra customisations')
        )
        self.options.add(
            OptionRecommendation(name='opt_kepub_delete_unmanifested', recommended_value=False,
                                 level=OptionRecommendation.LOW,
                                 help='Select this to silently delete files that are not in the '
                                 'manifest if they are encountered during processing. If this '
                                 'option is not selected, files not in the manifest will be '
                                 'silently added to the manifest and processed as if they always '
                                 'were in the manifest.')
        )
        self.options.add(
            OptionRecommendation(name='opt_kepub_hyphenate', recommended_value=True,
                                 level=OptionRecommendation.MED,
                                 help='Select this to add a CSS file which enables hyphenation. '
                                 'The language used will be the language defined for the book in '
                                 'calibre. Please see the README file for directions on updating '
                                 'hyphenation dictionaries.')
        )
        self.options.add(
            OptionRecommendation(name='opt_kepub_replace_lang', recommended_value=True,
                                 level=OptionRecommendation.HIGH,
                                 help='Select this to replace the defined language in each '
                                 'content file inside the ePub.')
        )
        self.options.add(
            OptionRecommendation(name='opt_kepub_clean_markup', recommended_value=True,
                                 level=OptionRecommendation.MED,
                                 help='Select this to clean up the internal ePub markup.')
        )

        EPUBOutput.__init__(self, *args)

If I add either of the following methods to the above class, I get the same exception thrown saying that there's no format plugin for 'kepub'.

    def gui_configuration_widget(self, parent, get_option_by_name, get_option_help, db, book_id=None):
        return None

    def convert(self, oeb, output_path, input_plugin, opts, log):
        super(KEPubOutput, self).convert(oeb, output_path, input_plugin, opts, log)