And he doesn't use a red saber, I presume?
Anyway, I expected the EU would've explained everything, but I'm not interested in the "in-universe" explanation (which sounds more probable than usual in this case) so much as what the people making the movie were thinking at the time.
The movie does kind of suggest it was all Luke's plan, going by Han's reaction to Chewie, who seems confident Luke has everything covered. I guess the whole Boushh reveal meant there was no earlier opportunity to bring it up. (Notice how Luke's plans tend to revolve around putting Chewie in shackles?) I suppose it's not much of a stretch to assume they deliberately kept 3PO out of the loop. I don't think he would've been much good as an undercover agent if they'd told him what the plan was.
Still doesn't seem Luke actually had a specific plan though. It seems he just gambled Jabba wouldn't kill the others before he got there, and that an opportunity for breaking out of there would present itself. I guess in a way that shows that the heroes were by then confident in their own heroics, which suits the feel Lucas wanted.