I have also been musing on that corrective elements should be brought back into UW photography, á la Nikonos, but a general afocal solution is great! I'm very interested in what the practical / theoretical limits of FOV could be. Also convex front elements of ultra widangles probably cause problems with fixing the lens. I suppose the internal and external corrective lens distance and co-axial pracements are very very important.
Keep us posted, and I hope some manufacturers catch on!