Just attach a small motor to one of the legs with a cam and just have it push off the floor a little each time it comes around causing the chair to lean back.
I love Monster Guts! I bought one of those a couple of years ago and bugged the poor guy to death and he was nothing but polite and helpful!I'm using the wiper motor from Monster Guts - great company to do business with.
Electric Motors for Props - 12VDC Wiper Motor - Halloween Prop Building Supplies For The Home Haunter - Monster Guts - Wiper Motors for Haunting Needs
For the rocker, I used a 5VDC supply and the motor is set for slow speed. Get the Quick Connect cable for the motor, very useful for attaching the power supply.
Sorry for the double post - hit the button too soon...