The External Accessory
framework is indeed for MFi only, but you have two options:
Jailbreak (I can't give you any advice there).
Use
CoreBluetooth
instead.
I've used CoreBluetooth in my own applications and it can get the job done, depending on how much data you're looking at transmitting. I don't know what an optoserial device is, how how much data you would need, but we're talking data transmission on the order of bytes efficiently. CoreBluetooth
operates with Bluetooth 4.0 LE devices, right now that is any iOS device released after (and including) the iPhone 4S.