A student from U.C. Berkeley approached me about using infrared for a forestry study, and we explored the vintage Nikon 8mm fisheye on the D70-IR. Regrettably, the projecting flange at the rear of the lens precluded using it on the EOS 5D-IR; I wasn’t willing to force the lens on to see whether it would destroy the mirror.
No evidence of hot spotting with the 8mm was observed, even at f/16. This might be due to older “less sophisticated” lens coatings, which are more infrared friendly than modern multi-coatings. Flare control is excellent as well, but sharpness seems modest, even at f/11. Still, it's a fun lens to shoot! By comparison, the Nikon 10.5mm DX fisheye fills the DX frame, but has hot spot issues when stopped down.
Nikon D70-IR + 8mm fisheye @ ~ f/16
(No hot spot problem whatsoever)
Nikon D70-IR + 8mm fisheye @ ~ f/11
(No hot spot, no veiling flare, a few flare spots)