500V unity gain buffer
This is fairly simple. Use an isolated supply to produce (let's say) +/- 15 volts, and tie its output common to your output HI. Then use any op amp you like (selected for output current), powered by this floating +/- voltage to provide your guard current. Since you can easily find AC/DC power supplies with 2 kV or more input isolation (try Digikey, for instance), this should be no problem.

EDIT - If possible, find an old-school linear supply. Modern supplies tend to be switchers, and the switching noise may in fact be objectionable. Of course, with careful attention to layout, filtering and shielding you can produce very clean outputs, but you do need to pay attention.

FURTHER EDIT - WRT the previous, you MUST use a dual supply. You cannot use a single supply with a rail-to-rail output: they just don't actually go to the rails with any current ability. And using a single supply with a virtual ground to connect to HI is not a great idea either, since it invites noise pickup on the virtual ground.