I would say no you cannot do this legally. The bus of the breaker panel may be covered and the breaker you are working on could be locked and tagged off, but the breakers adjacent to it are still on. Even though they probably have finger safe wire terminals there is still a substancial portion of the lugs protruding enough where you could come in contact with them.
I have only seen a few large distribution panelboards that have a true isolated can around the load side of the breaker with an individual door so as you could truly work on a connected breaker, most small ones have a simple front cover that covers the load sides of all the breakers, thus if you remove it you have exposed the live circuits.