Hi,
I'm developing something similar, but I used a big microcontroller (several I2C and mpu9150), did your idea of using AD0 as a select bit worked? Also i'm using mpu9250 and SPI Interface to read several IMUs at the same time, here main processor is a Cypress. Nice proyect, please tell me if AD0 can be used as you said, it's an very interesting approach!
Best regards.
Juan.