The short answer is that they didn't seem to have a plan for the death of a single astronaut, just critical mission failures.
As you will recall William Safire wrote a contingency speech for Nixon should the Apollo 11 mission end in failure. In his book "Before the Fall: An Inside View of the Pre-Watergate White House" (Transaction Publishers, 2005) Safire recounts a conversation with Frank Borman, an astronaut on board the Apollo 8 mission.
Shortly before the actual landing, Borman encourages the administration to make plans in the event of mission failure; something Safire says had not been considered. If the administration had not even considered what to do about mission failure until (roughly) a month before the actual landing, it seems unlikely that they had plans to handle individual deaths.
If fact, it's not until the 2000's that NASA commissions a study on how to store a body for return to Earth. Here are two articles on the current thoughts on the question: