Hi absentx, here are the answers to your questions:
-Does that rear circle looking thing above the rudder come off and allow access to any of the rear hull area? No it does not, it is the steering drum which connects to the rudder through a small scupper hole.
-Is that nice circular outline with the Hobie "H" in the back tray for crazies like me who want to saw into a brand new hull and add a circular hatch? Yes it is
You can add an 8" twist and seal hatch in that location. In older Outbacks that hatch came standard because it was needed to access the internal steering lines. With the new steering system on the 2019 Outback it is not needed. One thing we noticed with installing this hatch is that there is a good chance of cutting the rudder up/down lines during the install, so the line needs to be loosened before doing any cutting. We are working on instructions on how to do this which should be ready this week. I'll post them here as a reply as soon as we have them.-If the later is true, is there even any room under the hull at that point for storage or is it more compact now? Yes there is a fair amount of room in there. It's almost 5" deep in the center and about 2.75" on the edges where the grooves come up from the bottom of the hull, but it drops off deeper again to each side of those grooves. Definitely enough room to store some smaller items back there.
Hope this helps!