There are two types of devices that can be inserted into the small Montblanc pens. Both are so to say refillable cartridges that you can refill from an ink bottle. One has a screw/piston to suck the ink in. The other is basically a bladder inside a metal cartridge. There is an opening in the metal hull and you can pump the ink in. I prefer the screw type but the pump type holds more ink.

Since you are using already cartridges you might just squeeze some ink out of the cartridge. Additionally you could try to squeeze some ink out, maintain pressure on the cartridge and re-insert it into the pen. A similar procedure is recommended for shampoo bottles and the like. It works for those.

Or you simply put no cartridge in the pen while flying and only put it in when you are at the destination.

All of that said, I have several fountain pens by Montblanc, Waterman, Pelikan, Lamy etc and have never had the leak problem in flight. The Waterman Edson is supposed to have some leak prevention built in. An expensive proposition, though. Plus, my mother has one and it doesn't write very well and has a tendency to be messy even if just on her desk.

Would a nice rollerball perhaps be the answer to your troubles?