My idea for a Palpatine appearance would be after the Battle of Yavin, where he contacts Tie Advanced-riding Vader via hologram. Palpatine would inquire about the pilot and Vader would say he felt “a presence I’ve not felt since” (reused from earlier). One can imply that Vader is talking about the “son of Skywalker” and it sets the stage for their conversation in the next movie (where both guys know Vader is Luke’s dad). Distortion of the hologram can cover up whatever syllable mixing or speech synthesis is needed to complete the interaction.