In theory, AGMs do not gas and therefore can be put in places without the same level of ventilation as batteries that do gas. AGMs are still not exempt from code, though...if that ever becomes an issue for you.
I'm putting AGMs under my bed, but will still rig ventilation for heat dissipation. I'll have a 2000W inverter under there, too. I'm not concerned about the gasses as much as the heat during heavy charging and use. Although...all batteries are subject to explosion if a critical failure occurs. You makes your choice and you takes your chances.
I asked roughly the same question, and the answers were basically - "I couldn't recommend it but I can't say I wouldn't do it."
I wouldn't do this with any kind of non-sealed batteries without a fully-sealed case vented to the outside with a fan, rather than passive venting. That's just where my risk analysis puts me.
