It's still extra components (far more than just a FET) but it would allow you to run the MDBT42 off even a single LiPo battery and still get a higher voltage to light the LED.
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
Wow, those buttons look awesome! I might have to put an order in for some of those :)
But as you say, it doesn't look possible to easily change the LED :(
One more option is a 'voltage multiplier' : https://www.allaboutcircuits.com/textbook/semiconductors/chpt-3/voltage-multipliers/
It's still extra components (far more than just a FET) but it would allow you to run the MDBT42 off even a single LiPo battery and still get a higher voltage to light the LED.