Good question. I think at this point, the refractometer idea is out. It is simply too complicated, and we don't have the necessary expertise in optics and such.
We have had another idea floating around in our heads for several months, but have not had a chance to test it out. I am being a bit vague because I think that the idea is fairly novel. We will start testing this out soon and send out updates on our progress.
In terms of the form factor, we have been thinking of eliminating the wireless aspect at least at first. Instead it would be plugged into the Model-T sensor port, and receive power and comms over that. This would GREATLY simplify the design since it would not entail careful RF design and tuning, careful coding to minimize battery usage, or battery charging, which are each very tough problems on their own as we found out with the refractometer. This would also significantly reduce the cost as the entire device would consist of a few sensors, a microcontroller to read them, and an enclosure - easy peasy.