The debt to Jabba is entirely self-explanatory and needs no more explanation- it didn't at the time, why would it now? Even if it did, it could not be covered in the time of the prequel films, which would be set too early.
You don't NEED these minituae explored. That is not the point of the films. Han Solo became a classic character with NO in-depth exploration of his background, as did Vader.
I don't figure Solo will be in Episode III. As for Lucas doing something for the fans, isn't Boba Fett's appearance just that? Lucas has been quoted as saying he never understood Fett's popularity. I know the whole saga was suppossed to be written before the original trilogy was made, but I'm sure a few things were changed prior to the filming of the prequels. Why have Boba Fett in them at all if not for the fans?
Oh, the Prequel Trilogy is very, very different to what GL had in mind when he started writing the original films; only a few basic concepts have remained and so having Fett in as a sap to the fans is very possible. Although he did always have the vague idea that Boba was the 'father' of the stormtroopers (which when he wrote AOTC he transferred, for practical reasons, to Boba's father, of course)