You can just have your plugin create the ZIP file.
If you want to use the HTML2ZIP plugin to create it, have your plugin create the html file and call it like this
Code:
from calibre.customize.ui import initialized_plugins
for plugin in initialized_plugins():
if plugin.name == 'HTML to ZIP':
plugin.run(path_to_html_file)
Note that the initialized_plugins function will only be available in the next release of calibre.