After collecting dust for half a year, I revisited the Executive's again. Seen here is the Executives reduced to only the circuit board and the battery compartment. The ANC mics aren't connected. A 3.5 mm stereo mini jack is directly wired to the input, and the headphone jack in the ear cup is wired to the output.
Plugging any headphone into it will make them sound like the Executives; a bit hissy (would've been worse with the ANC mics connected), a bit louder and you'll get a unique v-curve EQ tuning.