Which isolated ground should I use for an RF can/shield over an optically-isolating component?
A number of considerations, having read the data sheet.

Follow fig 17 a lot closer than you are showing in your simulation diagram. 100nF on input and output supplies, RC lowpass filter with a capacitor across the inputs. Choose the capacitor value such that it does not impact your signal accuracy too much, while providing a low impedance at RF. At least 100pF according to the pages of hints an tips in the data sheet.

That part is designed for a colossal transient common mode rejection. As such, any attempt to put a can round it may well degrade that. With a good layout, so short leads between the IC supply terminals and the 100nF, short leads to the low impedance 20ohms and a capacitor across the inputs, the IC will be very hardened against RF interference. Follow the tips about how to connect the -ve signal input, with respect to other connections to that node.

Try the part out without kludgy metalwork first, but with good layout and appropriate capacitors. You can't screen your way to success if your layout is bad, or if you are lacking the decoupling suggested in the applications info. Once you have the latter, seperate screening is rarely needed.

Caution, 1000v across R1 is 10 watts, 5W average for a sine wave. Make sure it can dissipate that, while the tempco is within your accuracy specification.