From a review of the 1906 Locomotive Builders Dictionary¹ it seems like the pilot, and also likely the tender couplers on 622 should not have draft gear per se, but would have been mounted in cast coupler pockets, without any springing at all. These were very narrow affairs, barely wider than the 6-inch shank of the coupler itself. So, if I am to use Accumate couplers on this engine, a little compromise is necessary.
None of the photos I have shows the coupler pocket in sufficient detail, and so, I found the Tower Pocket Coupler from National Malleable Castings Co (page 237) that seems to fit. It serves as an inspiration more than prototype, as it needs a much wider opening to enable the Accumate coupler to slide in from the front. Hopefully nobody notices the compromise.
¹Thanks again to Don Ball for pointing this resource out to me, and to the University of Wisconsin for keeping a copy, and to Google for digitizing it!