FUNimation DVDs having Japanese extras isn't too uncommon. "Summer Wars" had a bunch of interviews with the Japanese staff, one of the "Lupin" DVDs had a film of a "Monkey Punch convention panel (creator of "Lupin the III for those not in the know). I think what it comes down to is that if there's some good extras from Japan that FUNimation thinks they can feasibly pay for, they'll go after them. Otherwise, for the most part, whether they can get Japanese extras or not, they'll stick their own in-house extras in there to appease fans, and it's especially important when they can't get any good extras from Japan or there aren't any available at all.
With the case of "One Piece", all we're getting are commentaries from the FUNimation staff probably because there aren't any extras from Japan that FUNimation thinks are worht purchasing. There might not even be any good material from Toei Animation at all. Although I know that at least one of the "One Piece" DVDs of the 4Kids version had production art as an extra.
You know what would be a good extra for one of the DVD sets? This:
Unfortunately this is just something filmed by a fan so I doubt this could be something that could be put on an official product.