Yes, Don Knotts did voice work for
The Griffin and the Minor Canon, which featured David Selby as the voice of the Griffin. Each actor recorded his/her vocals separately, so, while they may have been in a scene together (I can't remember off the top of my head), they were not in the studio together. Don Knotts and Soupy Sales have the distinction of being the only two actors in the project whose animated characters are drawn to resemble them.
WV Public Television has pretty much gutted their
official site for the project. There used to be photos of Sales' and Knotts' characters somewhere on the web. I'm not sure if they were on that site, but I don't have a link handy to a site that has them.
There's another funny story about this project. The producers were looking for information about what exactly a "minor canon" is so that they could include it in the teaching materials. Their web search turned up my Selby site because I had read about the project in WV newspapers online and posted a blurb about it on the News page. At some point, they showed the site to Selby, and that's how he found out about it. (He's not exactly an avid web surfer.) The producer/animator also contacted me as the film was being released to thank me for the publicity, and we've stayed in touch. Are we all bored yet?