The rear seal is possible I suppose, but I'd say unlikely, Mike. I say that because except for the area I already mentioned...where the cap connects to the block, there is no pressurized oil going to the rear seal area. It is hard for me to see most of a quart flying out through the rear seal even if you forgot to install the cap bearing. (we can see the bearing for the block is in place). Gravity would be your friend there and the oil would just dribble off, anyhow. Oil galley plug, camshaft plug...yes, a failure there would pour a lot of oil out, but I think you would have seen a problem when you put those in.
One thing I did notice is the top rear bearing sits more below the surface of the block than I'd expect. Could you post a picture of the cap with its bearing installed?
Depending on the clearance you left at the rear main bearing (hopefully .001" or so), not much oil will squirt out of there normally. A lot of blow-by will force more than normal oil past the rear main seal, but your engine wasn't running.
And you already installed the flywheel, so I'm assuming you put thread locker on those threads. You can get a bunch of oil through those holes if you try this before putting on the flywheel, but I'm certain you know that.
Did you feel the back side of the head?
