So, I've figured out where the config for tinyMCE is in 2.3.2 and see that iframe has been added as standard, thank you.
Now I would like to add support for the onclick attribute, specifically for the <li> tag so have put this in mysite/_config.php
HtmlEditorConfig::get('cms')->setOptions(array('extended_valid_elements' => 'li[style|id|onclick|title|class]'));
but this doesn't seem to work. Am I on the right lines? I'd rather not edit LeftAndMain.php