I know you have stated that you would like a small mic to go in the ear cup etc, but I have used my technique of a personal minidisc recorder a 3.5mm to 3.5mm cable with a 1/4" jack connector on the end. This has worked fine in the following aircraft:
DA40
TB20
C150
C172
Piper Aztec
Piper Chieftain
Piper PA28
And doesn't interfere in the slightest. I have to then match up the audio with my film (because the hd camcorder doesn't have mic input)