While there are rumors of a USB HDD for the PS2, we will have to wait and see. I don't think the add-on disc is very likely.
Unfortunately I think the PS2 has nearly reached the end of it's productive cycle... we are going to be stuck with Need for Speed and GrandTheftAuto rehashes until the PS3 comes out.
Don't get me wrong, I love my PS2... I was even playing it before the Xbob was released. You don't see PC gamers using the same system they had 5 years ago. However, the PS2 is extremly outdated right now and adding the cost of a hard-drive to an outdated system does not make sense.
In short, we are not going to get any major updates, maps nor a sympathetic word from the developers. Word of advice... if you need a Jabba's Palace fix, stay good friends with your Xbob friends and "borrow" time on their system till we are back in the modern world.