I have to admit, at first, I was a bit wary of Usopp. He seemed to be on a completely different show to most of the other characters. It wasn't until Arlong Park, that I realised somewhere along the way he'd completely won me over, and he's now one of my fav characters. :D And, yeah, Oda is a saucy thing. He seems to have created this wonderful balance of obvious fanservice (Nami's skirts, Robin's barely contained breasts, Zoro's constant shirtlessness) and strange seemingly-plot-driven stuff that he lets the audience read as pervy, without outright stating it to be so. Oh, I loves him hard.

You know, now you come to mention it, the only other fic I can think of with gratuitous One Piece tentacle-action is Squeaky Clean (http://community.livejournal.com/onepieceyaoi/285286.html) by alita_b_angel, which needs to be read and worshipped for the perfection it is. Other than that, the closest my mind can come up with is Smoker/Ace logia smexing (which, as you pointed out, is canon XD). There must be more than that, surely? I've been out of fandom, and teh Intawebs, for a while now, so maybe I missed some? *is hopeful, wants tentacle fix*