I recall reading that Doug Trumbull claimed that they'd used Nikon lenses for a lot of 2001 shooting, and that they'd kept it quiet so as not to anger Panavision. A quick Google serch came up with this article: http://articles.latimes.com/2003/nov/27/entertainment/et-hal27/2. The two salient paragraphs here say:
"In a detailed e-mail on the subject, Trumbull said, "Any claim by Kirk that the Fairchild-Curtis lens was used for the HAL [point-of-view] shots is just not true." He contended that the Fairchild-Curtis image would fill the entire frame of Kubrick's 70-mm movie, whereas the point-of-view shots of HAL are round and have vignetted dark spaces to the left and right of the 70-mm frame. Therefore the point-of-view lens would have to have been smaller."
[...]
"Though not taking sides, Frewin, Kubrick's assistant, recalls that the director used a 160-degree "mapping lens" borrowed from Fairchild-Curtis for HAL's point-of-view shots and later returned to the optical company. Frewin said Kubrick also used a "normal, commercially available Nikon fish-eye lens" for other HAL shots. The debate underscores not only the respect experts long have held for Kubrick's filmmaking techniques but also the enduring fascination moviegoers have had with HAL." In another interview (http://parallax-view.org/2012/02/11/breaking-new-ground-has-always-been-in-the-medium-itself-an-interview-with-douglas-trumbull/), Trumbull says: "There was a commitment to Super Panavision, they called it Cinerama, but he was feeling kind of frustrated with the lenses available from Panavision for that medium. So we started experimenting with Nikon lenses and he found out that the Nikon lenses, which were designed for the 35mm slide format, actually had a field of image at the back of the lenses that was enough to cover the 65mm film format, which was just a little larger. And so, even though we were forbidden from publicly announcing that Nikon lenses were being used, we were using them all the time. But he was involved with that. And he would shoot shots. We had these Panavision cameras with various different lenses, some Nikon, some not, shooting the Discovery spacecraft for instance, and he would test everything until you were ready to just tear your hair out, because he was so meticulous. The shooting ratio on 2001 was 200-1. That is like way out there. And the difference between a 60mm and 55mm lens was big for Stanley."