Is there a way to make a command (not a verb) appear for an object which is either in the player's inventory, or in a room--while only in that room?

Not sure exactly what you mean by "appear", as only verbs appear (display verbs, which when you click an object link) - commands don't get displayed anywhere.

But if you do mean command, you can add a command to a specific room and it won't work anywhere else.

Alternatively, any verb you add to an object can run a script - use "if" to check what room the player is in, then act accordingly.