Greetings,
This is a really interesting and challenging product. Due to it's size constraint it becomes further involved. This design involves ADC/DAC, Bluetooth Classic, Digital controller to be able to do the Analog Audio (3.5mm jack) -> Digital -> Transmit over BT -> Receive over BT -> Digital -> Analog (3.5mm jack). And since you can't draw enough power from the 3.5mm jack it would need a tiny rechargeable battery and associated circuit to charge it, and get a stable voltage out of it. Also the charging is not done through the standard micro USB - it is through the audio jack only - which will further complicate the design.
Overall fairly involved design, plus the PCB design & layout, firmware coding and then you would need few working protos at the end. I have good expertise with such products - recently completed one baby wearable involving sensors, BLE, rechargeable batteries. Please see my profile for more details.
It would be fun to work on this however as I have mentioned in my bid - the time and effort involved are much higher. So let me know if you are open to a higher budget of the order I have bid - it would be great to discuss out the details.
T & R
Alok Pugalia