I have attached the plugin here which I hope is okay.
I don't think they really 'hack' the code and bypass the index.
The plugin simply adds a <link> statement to the head section of the site which points to a cached file ( it one exists ). I don't think this is really any different from outputting any other <link> statement in the <head> section. Joomla core will create the <head> section based on the plgins etc but when this is rendered the browser itself has no concept of joomla framework...
Do you think that css.php should try and get an instance of the joomla framework when it is called? Is this possible from a dynamic file called from a <link> statement? Do you want me to post a link to slave site which is demonstrating this issue?
Anyway, all your help is much appreciate.
Adam