1200 amp 480v, panelboard, main circuit breaker
Fed from utility transformer with only high side fuse
The arc flash rating of the line side of the main is about 80 cal/cm2 based on 2 second clearing time (since the high side fuse will have a clearing time of never).
The panelboard is the type that has seperate covers for the main circuit breaker and feeders, and therefore work could be performed on feeder breakers without removing the cover over the main lugs. It has a servance entrance label.

If the main breaker was turned off, could work be performed on the load side of the breaker, such as adding a feeder breaker?

Depends on the breaker type. Just opening a breaker is not sufficient for isolation purposes oer OSHA. A power circuit breaker needs to be racked out to the disconnect position and locked out for proper isolation.

Main

It's a molded case circuit breaker that is LOTO.

If the main is locked out and is in a separate compartment, there are no exposed energized conductors. You can install breakers or whatever. However at 80 cal/cm2, the problem comes when operating the 1200 amp main and verifying the absence of voltage.

Operate the breaker remotely.