i've actually seen something like this. i was in a civic del sol vtec, and the owner had a ricer button to manually control the vtec. there really was a considerable difference in exhaust tone for sure.The mivec system is basically turned on and off by 2 solenoids one in each head. When the solenoid is engaged it allows oil pressure to flow into the mechanism that engages the high lift rockers. At least that's the way i understood it works. When it engages there is an audible difference in the exhaust tone.
anyway, great job on the swap! please keep us updated