I think much of the cast, if not just the mains, got a chance to take something home. Two others I can think of is Sam Jackson has a hero Windu hilt that he's shown off before, and Christopher Lee kept one of his in a vault and it was auctioned not too long ago.
Lucas said that Qui-Gon didn't perform whatever technique that retains your identity perfectly or he didn't learn the exact way to do it, so that's why you only hear him. Presumably the three of them (Yoda, Obi-Wan, Qui-Gon) perfect it during the intervening years.
He was supposed to be talking to Yoda in the scene in ROTS, when Yoda was meditating after the birth of the twins, but Lucas cut his lines. I don't remember if he actually recorded them or not, but I was thinking that he did. I do know that Neeson appeared in TCW Mortis arc.