Obviously I’d have preferred to have heard Guinness’ voice in the Sequel Trilogy, Tantive3+1.
But like Fan_edit_fan posted, JJ seemed to be throwing around ideas last minute or off the cuff throughout, including the Emperor’s appearance in TROS according to Ian McDiaimid himself.
The TROS scene with Ewan’s voice as Kenobi: https://www.youtube.com/watch?v=KL3ZLxegIyE (3 minute video). Beware: contains TROS!