Well, didn't I display my ignorance in my previous post. But your sentence saying "Your post shows just that you dont know much about hosting or ispconfig" was apparent enough to all without the unkindness of bluntly saying it. Your post would have been just as useful and informative without underlining my shortcomings.
That said, I did start it with unfounded observations of my own, which, based on learning from your brief explanations, I withdraw unreservedly. ISPConfig is evidently clever, well thought through software in ways that I am only beginning to learn and understand. I'm working on it.
My sites were set up to use the default of fastcgi and now that you've guided me in the right direction it is plainly obvious where the open_basedir settings are written - in a hidden fcgi scripts file just above the site's document root.