Overpressurizing a cream whipper?
Charging a whipper two or three times is definitely not going to make it explode; for certain lower-density preparations you're supposed to charge it twice, but even if that weren't the case, consider that one of the chargers holds only a fraction of the volume of the whipper itself (8 g, to be exact), so if the chargers don't undergo explosive decompression then it's definitely not going to happen to the whipper.

As for it possibly turning into butter, that's a more realistic possibly depending on what you're putting inside it.  I can't verify whether or not it happens with cream since I've never tried it and probably never will; I wouldn't recommend it, since whipped cream gets very dense to begin with.

But honestly I think your biggest problem is going to be that whipped cream dispensers aren't designed to dispense upward.  The instructions are very explicit that you have to hold it upside down in order to dispense; if you hold it right side up then it's just going to vent the gas and make it impossible to get the cream out with opening it up.

Simply stated, the cream is much heavier than the gas, and I believe that whippers don't actually achieve saturated vapour pressure inside.  That's why the gas will always eventually end up on top no matter how you orient the whipper; the key to getting the cream out is forcing the gas back down through the cream by depressurizing it, which is why you need to hold it upside down.

So unless you want to eject it from the bottom of the cake, I'm sorry to say that I don't think this is going to work for you.

One alternative you might be able to try is to pre-whip the cream with a stabilizer or make a fondant, then put it into a hollow tube and eject the gas from the whipper into that tube.  That way you can guarantee that the "liquid" is actually in the path of the gas, and the gas should propel it upward.  I emphasize should; it's going to depend on the density of what you're trying to propel and how good your seal is, and you'll probably have to experiment a little.