Would'nt it be a good idea for the library to let the slaves know it is in control over the bus after a reset by sending a few clockpulses ?
I would give it a try ... what about something like this in setup, just before Wire.begin()?
pinMode(21, OUTPUT);
for (int i = 0; i < 8; i++) {
digitalWrite(21, HIGH);
delayMicroseconds(3);
digitalWrite(21, LOW);
delayMicroseconds(3);
}
pinMode(21, INPUT);