Why is it not meaningful? "Isolated systems" seems meaningless - there is no objective cutoff where a gravitational system becomes "isolated", except perhaps in the sense of "non-intersecting light cones".
I have read all of your comments and not one of them actually says anything concrete. It's the exact same vague objection repeated over and over again.
Please explain exactly why you think calculating the total gravitational potential energy of the entire universe or a well-defined sub-volume of it is intractable. Feel free to use arbitrarily technical mathematical or physics language, just please stop being vague.
