This year I hunted out of a Misty Mountain RCH + Dryad Drey. Similar effect to your SitDrag. I climbed on the RCH, deployed my Dryad at height, and stowed it before descending.
I'm no expert but I understand that RCHs are generally built to the same safety specs, but differ on various factors - weight, comfort, accessory points, etc. Since you're likely augmenting with a battle belt or MOLLE on your sitdrag you won't need a bunch of extras on your RCH. I would recommend a linesman's belt, though.
The belay loop you're seeing on the BOD is meant to be joined to the waist belt with the carabiner to which you attach your lines, or to the lines itself. There's some discussion in the rock climbing world regarding nylon-on-nylon friction vs carabiner-on-nylon friction and which attachment method is better for your style of climbing. I won't summarize it and suggest you research that point on your own.
I put a 2' sling on my RCH. I put my climbing/rappel line directly to the RCH, and clip my hunting tether to my sling. At height I tend some slack into my rappel line, install my Figure-8, and clip that to my sling as well, with my friction hitch "third hand" below the Fig8. I like it, but 2' is a bit too long, go with 12-18".