With only a slight modification to PWMServo.h the PWMServo_v2 library found here: http://arduiniana.org/PWMServo/PWMServo_v2.zip works just fine with the ATtiny84. After unzipping the folder open PWMServo.h in a text editor. Towards the top of the file, at the bottom of a bunch of 'ifdefs' after the #else label, change the lines #define SERVO_PIN_A 9 and #define SERVO_PIN_B 10 to #define SERVO_PIN_A 6 and #define SERVO_PIN_B 5. Then of course save the file. Now you will access to servo outputs on physical pin 7(SERVO_PIN_A) and physical pin 8(SERVO_PIN_B). I'm using it with the ATtiny84's internal 8mhz clock and it is working great.
This also works and maybe it is a more proper mod. Make this one of the #elif's before the #else label:
define SERVO_PIN_A 6
define SERVO_PIN_B 5
I used this and got it going very nicely indeed. Do you know if there is a way to drive more than 2 servos on the 84?