Anything you pull from properties.xml is a system defined value ; the "Idea" that Rob pointed you to explains that nowhere in properties.xml is there a distinct system-setting for a "XOG only URL".
However, even if there was a value in properties.xml then this does not save you any development work as you would still have to edit all your thousands of existing GEL scripts in order to change the attribute that you are picking up from properties.xml. This is of course, exactly the same amount of work (perhaps even less work?) that you have to do to hardcode the value in the GEL script.
However, you have now realised that you would perhaps like to parameterize the XOG url (which I suggested above you might like to do) - then you could just update parameters (on the database) when moving to different instances. Even better would be a soution that pulled the URL from some common area under your control - perhaps a bespoke database table/column (but could be anywhere that you able to store a value) - this solution would of course involve you writing even more code to retrieve the URL (but that would be common code that you could paste into all your GEL scripts in place of the code pulling the value from properties.xml)
Of course this is all up to you how you do it (personally I'd have a common parameter to my GEL scripts I think if I had this requirement), but then again personally I would never have picked the URL from properties.xml in the first place as I'd would instinctively use the endpoint="internal" (which amounts to the same thing as reading it from properties.xml so still would not help with your original problem).