There are rumors the Route 66 ep "I'm Here to Kill a King" was intended to air on 11/22/63, though I haven't been able to verify that (for those who haven't seen it, umm... while it's set at Niagara Falls and involves some arab leader and a twist that didn't occur in the real event, it involves a philosophical wanderer using a rifle to shoot from a grassy hill at around noon). That episode was postponed, airing after the series finale in 1964.
To answer DeepSpaceWine's question: "I'm Here to Kill a King" was originally scheduled to air on 11/29/63, the week after the Kennedy assassination. It was pulled for the obvious reasons you described above. The "Route 66" episode which was supposed to air on the fateful day of 11/22 was "Kiss the Monster, Make Him Sleep" guest starring James Coburn. This episode was later aired the following winter. An episode titled "A Cage in Search of a Bird", guest starring Stefanie Powers, aired on 11/29 in place of "King".
I can find no evidence that "I'm Here to Kill a King" ever aired during "Route 66"'s original run. I've heard different claims that the episode aired on Feb. 28 or on Mar. 20, but TV listings published for that date show different episodes scheduled to air. However, the episode did make it into syndication, and the cable station Nick at Night included it in an "Evil Twin Marathon" they ran sometime in the mid-80's.