I've been doing this for years for flight testing.
Small tie-clip microphones inside the headset work well, the Maplin telephone pick-up coil on the outside of the headset (or with casing removed then a bit of mic-sock-tape around it inside) works much better because it is just picking up the headset side-tone, which is exactly what you want.
True cockpit noise will swamp a microphone actually in the cockpit.
The other advantage of the Maplin pickup coil (as I discovered to my embarrassement a while ago) is that it still works after it's been through the washing machine in your flying suit pocket.
G