Puggo - Jar Jar's Yoda said:

Hey that greenscreen of George could be useful in some projects here.

Is it possible that they just filmed some blank shots of the desert in Tunisia during Ep.2, so that they'd have backdrops of Tatooine for use in the greenscreen shots of Ep.3?

Shooting background plates "just in case" has been a common practice for a long time. I first read about it being done in the making of book for the first Star Trek movie. They shot plates of the V'Ger set from several angles in case retakes were needed after that complicated (and dangerous) set was taken down.

According to the DVD commentary tracks, there are a couple shots in Episode II and III that use backdrops of sets they would have had to rebuild otherwise.