In short. QrCode does not support byte input. As far as according to ISO. It is totally char based.
Even though we do convert char to byte array according to char table, but if you check any char table, most them contain non solve able special char. Decoder will always assume income byte as part of char table, and turn it into
question mark if non solve able.
If you have total control of decoder, then its possible. Grab byte before decoder turn into char. That means you have to have a QrCode decoder that's under your demand. That's not easy. Especially for paid solution. If you want to do that, you can. Put
some time on ZXing's decoder, and break it down. Grab byte array before char decode.
Same at encode side, ignore char encode, just put in byte array. That's what I want to do at later stage, I don't have time set up yet as been busy recently. For this project to include decoder will take longer than optimize current encoder.
If you can get decoder going, I can help you with encoder.
QrCode specification is under our home page. You can download and give it a quick read for DataEncodation.