kivig:
On the other hand I suppose I could use a few Attiny85 to offload PWM if they support servo library functionality...
How are you going to communicate with the Attiny85? Via serial? You haven't really solved the problem.
If you use the hardware timers though it could be OK (hardware PWM). The servo library is designed, as far as I can see, to support any pin, which is why they intervene with an interrupt handler rather than using hardware PWM. You have 6 PWM outputs on the Atmega328, so unless you need more than 6 servos, the main chip should be able to do it, with careful use of timers.