Well, if your using an IMR battery (which is what I'd recommend), they will not vent with flames or violently. At most they will leak... If there were any flame it would only be the result of the high current/temperature melting the wrapper but wouldn't be something that would knock your teeth out.
If a Li-ion were to violently vent, my guess would be the gasses would escape through the operating panel side (being the weakest point & place for oxygen to enter).
Now I'm curious and in the near future will try to do a bit of R&D and purposely cause a battery to vent in a mod
