What I was wondering is if it would hold itself there...
No, but glue will.
The two things I fear most about doing this is
1) not angling it right and it not being able to line up and complete the circuit.
2) sins the chip would have to be at an angle to fit in the helmet it would just slide out.
Well, these are problems that can be solved, but they are also reasons why using a reed switch is unnecessarily complicated in the first place. Closing the contact mechanically would be a lot easier and most likely, as a side-effect, hold the chip in place.
As far as keeping the chip in, you're already using magnets to complete the circuit so just have the magnet contact a metal anchor on the chip to hold it in?
The chip is the magnet.