dumb rule

Not allowed as well as not the best idea for today's game.

People see Doc around the games, or any NBA coach, and they think back to their high school coaches or even small school college coaches and think that "Wow, any good player could do that."

Except coaching has expanded to be so much more. Film review, statistical review, specific game planning, personnel management, charity appearances, pre game scouting, long term scouting, draft scouting, running practices, running team meetings, running player film sessions, setting goals, implementation of defensive and offensive systems, creation of a playbook. These are all things that happen outside of the 3 hours, 2-4 times a team has to play and a coach has to "coach" a team.

In real coaching, in game coaching is the smallest part of the job. A player just can not do both jobs anymore.

Can't do it officially, but...

There's nothing stopping a player from doing a lot of coaching, that is development work, with one or more players on the team. Officially, he's still a player on the cap and taking a roster spot, not officially a coach.

See Juwan Howard.