I know two things that would get you halfway there.
1. Hordespawn. The code for func_hordespawn is both in Chapters and RMQ. Originally from Scourge of Armagon, IIRC. You would have to expand the code so the spawned monsters use a randomly selected classname and spawn function (out of a list), and basically set the count or duration to infinite. Would do what you asked, but it's a point entity, not a box. Simply continuously spawns (spams?) monsters in a certain interval and basically swamps the player in monsters. Infamously used in some Quoth maps to hordespawn Vorelings.
2. Pineapple's Randomizer. Part of RMQ (don't know if this was released standalone). This requires pre-selection and placement of the monsters you want to spawn. This would work if you only wanted a limited amount of monsters to spawn. It is more aimed at (pseudo-) randomly populating a map every time you play than continuous spawning.
This isn't quite what you asked for, but you could probably use it to get there. You didn't say what you wanted it for, though.
