MDBT42Q breakout - LED + opendrain question

I definitely wouldn't recommend that at all - it could cause lasting damage to your board.

What's happening is when the port is open drain, it's being pulled up to 7.2v by the LED, and the only thing stopping the MDBT42Q from being fried is an internal protection diode in the chip (the IO is only meant to have 3.3v on it).

I'm afraid all I can really suggest is:

• If you can get into the button you have, see if you can replace/short the resistor on the LED, or replace the LED completely. It's what I've done on https://coolcomponents.co.uk/collections/component-buttons/products/big-dome-push-button-red before
  
• Add a FET as you suggested (or there are certain transistors that contain resistors too, so can be wired straight in)
  
  

I think those are probably the simplest options. On the Puck.js we have a built in FET, but then that lacks the voltage regulator so would have to stay running off the coin cell