Hi,
Actually the output plugin works well for this as it changes nothing at all in the current ebook files inside Sigil as the only access is via a book copy but does make the entire ebook (less mimetype) file available.
Now if you wanted to do something special with the epubcheck output or wanted to make it easy to highlight text needing work in files or something along those lines, then a separate plugin type for validators with expanded capability would truly be needed.
Basically, the same approach used by Doitsu can be used to invoke kindlegen or any other separate tool. So if Doitsu or any other plugin author wants to build a Kindlegen plugin, say maybe with a search to find the kindlegen executable on the users system or a filedialog to allow users to select it and saved preferences for settings and options for kindlegen that would be a wonderful thing!!! ..... Hint Hint
KevinH