So far after some research it seems that the most elegant way to do this is to use 'before_body_end' reference. I found this article and recreated all steps exactly as described (except I'm using the 'community' code pool) on a fresh clean default Magento CE 220.127.116.11 installation. But nothing happens. The extension is enabled in 'Configuration/Advanced', cache is cleaned, no errors at all... The custom phtml isn't loaded.
Can anyone describe (or refer to) what steps should be done to make it work?
Maybe there is another way (not with 'before_body_end') to make it work?