Personally, I think it would be hilarious for Lucas to constantly say he has no plans to do 7-8-9 and then turn around and do an episode -2, -1, 0 prequel-prequel Trilogy that focuses on Palpatine and how he got his Sith powers.
Episode 0 could give us the long-awaited birth of Maul!
Plus with a prequel-prequel trilogy, Lucas can start saying stuff like, "oh no - this whole saga wasn't about Vader/Anakin, it was always meant to be about Palpatine..." and "what I meant by last SW movie was that Vader's SW Story would be over".
Um... I'm gonna vote No
to proposition "Make a prequel-prequel movie" - bad idea, George.
A Jedi v. Sith movie set 88 years before TPM flies in the face of the "The Sith have been extinct for a millenia" line from episode 1. The whole thing would end up being lame...