Sure, as long as you don't get zapped. A rig like that could work kilovolts off ground. "Optocouple" the waveforms back to ground by photographing the screen!
The last case, regular scope without a ground plug, will still have some capacitive coupling back to the AC line, so some hf ground-loop noise can still happen.
But, in all cases, only one floating common exists. The Tek TPS allows four separate folating signals to be scoped simultaneously.
John