I speculated elsewhere on these boards that once the ladychimp approach was ditched for ROTJ an attempt might have been made to take the shape of the Emperor from ESB and render it into various masks and make-up combinations.

Famously Ian McDiamid was not the first choice for the role, it was to be played by an older actor who became unavailable at the last minute so Ian (who was the second choice) was used instead.

Perhaps single masks were considered to reduce the make up chair ordeal of the older actor (the name of which escapes me at the moment)?

As I say all speculation but just as Vader concept masks were recycled into The Death Star Droid in ANH, it’s possible the abandoned mask designs were redressed to be background aliens in the briefing scene.