Woah.
Yeah, the plexi stuff, isn't that some sorta of safety standard? I remember reading about that somewhere, definitely before my time...
If I had to choose, I'd rather use an isolation transformer than ground lift the scope, but again best not to propagate safety risks.
But what I really want is a P5200 or someone to nick a circuit so I can DIY one
The other option is of course matched HV probes. Tek used to (still does?) sell probes this way. Then you do the invert and sum trick across the two channels. Not as cool as the above though, and cannot really be used on PSUs where you want a floating measurement.
Seriously though, ask Andy. He satisfied himself and whomever was working with him, so thats worth something I'd suspect.