@Doitsu,
The next release of Sigil will include a "validation" plugin type. If you use it then you can set the validation results in the plugin and they'll be loaded into the validation pane in the main window. Just like what happens when you use FlightCrew. This means the results won't disappear when the plugin runner dialog is closed and you can click on the entry to go directly to the associated error line.
Here is a quick example of how you'd use it in a plugin:
Code:
def run(bk):
bk.add_result('info', 'Section0001.xhtml', 1, 'info stuff')
bk.add_result('warning', 'Section0001.xhtml', 3, 'warn stuff')
bk.add_result('error', 'content.opf', 4, 'error stuff')
return 0