From the official baseball rules (http://mlb.mlb.com/mlb/downloads/y2014/official_baseball_rules.pdf):
"Players in uniform shall not address or mingle with spectators, nor sit in the stands before, during, or after a game. No manager, coach or player shall address any spectator before or during a game."
That seems to separate "players" from "spectators" pretty clearly, though neither term is defined in Rule 2.00 (Definitions).
The definition of Interference (Rule 2.00 - Definitions) says:
a) Offensive Interference is an act by the team at bat which interferes with, obstructs, impedes, hinders or confuses any fielder attempting to make a play.
Reaching into the stands is done at the player's own risk and no spectator interference can be called, but players in uniform but not currently active (say, on the bench or in the bullpen) are still players and I believe can still be called for Offensive Interference.
Therefore, I'd conclude that a player or coach in the bullpen may not interfere with a defensive player attempting to make a catch while reaching over the wall. If the bullpen had spectators in it for some reason (stand overflow, maybe?), the spectators could try to catch the ball, but you wouldn't have players/coaches in the bullpen then.