First I want to state that nginx is not less functional then apache in most cases.
My target was VPS oriented hosting server with control panel.
My intention was eliminate mail server usage (thanks to gmail) and totally replace apache+libphp with nginx+php-fcgi-fpm and not to add nginx as reverse proxy.
Currently I am successfully running desired configuration and I am more then happy :-)
Footprint, security and speed are far beyond my expectations.
I was able to add to ISPConfig, support for nginx, with as much features as majority of websites would require.
I hope to find some time and cooperation from ISPConfig maintainers, to share my achievements with others, soon.