It would definitely be next to impossible to put the original ROTJ Jabba in the scene digitally using just the ROTJ footage, but a way in which it would be doable would be shooting a bluescreen puppet. Luca$film should have done that, I'm sure they still have the original Jabba puppet in storage somewhere, they could have just shot the scene with it, with Jabba being on some kind of antigravity platform, and as Han walks around him, Jabba could have just turned to keep facing Han and no stepping on tail would have been necesary. But well, the Jabba scene is redundant anyway...