What items were installed prior to rear undercoating?

Besides the rear axle bump stops, gas tank, sender and fuel lines?
Also for certain cars the ECS system would be installed too. ANything else?

Brake lines were installed afterward from what I've read and noticed on my car.

sway bar and mounts (if your car had them)....

If the car had heat shields (like E Bodies), they were on prior to undercoating.

Just about everything, but it may depend some on the model year & assembly plant.

shocks were. I dont think the seats were, I belive that that holes were taped over

Seat belts were already installed as the tips of the bolts are usually undercoated

Any body plugs and bumper brackets which means bumper also as it was installed as unit

Rear axle and springs were not installed. There was undercoating overspray behind my front spring hangers. Agree that seat belts were installed and had overspray on the anchor bolt ends.