Great to hear confirmation of this. As a big Harry Shearer fan and some one with more than a little exposure to SW (from my childhood obsession from '77 to the early '80s to my work on dozens of SW books and comics in the late 90s) I began to suspect this in the late 1990s. I confirmed it a few years later directly from Mr Shearer via email.
As there wasn't any mention of it online at the time I created a Wookieepedia page detailing all the information about 5 or 6 years ago, including the dialogue and characters, but it was flagged for original content. Despite this it stayed up until a year or two ago. Nice to see the information finally get out there.