We have an interim solution for a swiveled seat foot support, and it's an unlikely one.
This aluminum step is a $25 Harbor Freight atrocity
(edit: inflation is a bummer - it is now $30) that I picked up a few years ago. I use it a lot in Canada and I tend to leave it laying around. Not a big problem if it sprouts legs and runs away from me - it's cheap.
We used it to try to get a feel for what kind of support we'd like for the swivel seat (height, surface area, etc.), only to realize that this piece of junk itself is actually quite useful in this application. With its clunky legs folded beneath it, it provides an inclined surface so that one's ankles are not over-extended. Plus, it scoots under the trailing edge of the swiveled seat when not in use (I have to keep the seat pushed aft like that so that (a) our front seat dog bed fits
and (b) I can reach the Lagun table when it is mounted here
I wouldn't have predicted this, but we are going to put it into use here to see if we like it. We had been carrying it with us 100% of the time anyway, but it had been living in the ATV bag that we mounted on the roof