As far as I know (waits to be told different) there is no difference between the rear subframes of the ST and G/S, so you can use the G/S single seat, rack etc on the ST. and if you look at my ST on the 1st page of this thread, you will see I have fitted the early style covers to my bike.
Yes, these subframes share the same part number. Also, I stumbled across a photo of an otherwise stock looking R80ST wearing a G/S solo seat and rack.
No idea who owns it, but you can check out the rest of the photos here: https://www.smugmug.com/gallery/2092986_pCjH3#107961338_oBHvg By backtracking out to his front page, you could figure out who it is or send him an email.
Time this was on the front page again, in case we have any new owners dropping by.... Right as I was asked, here are some shots of the screen on my ST..
Gidday from down under. I have a 84 St , red #6056628, 78000 miles at least 5th owner. A going concern used frequently. along with Suzi DR650 Cheers U