If 4kids would have you to believe their "I hid everything I own in one piece…" line. XD
Well, it would be ironic if it was a pun on Roger's part, but we'll find out at the end why its called One Piece.
I always thought it would be funny if it was something like "One Piece of history" or "One Piece of (Roger's) memories", but I doubt it's so simple.
And are you referring to Onigiri? Onigiri isn't a type of sushi, it's a riceball. But literally, it means "Demon Cut". Oni = Demon, Giri = to Cut.
I think the confusion stems from many people thinking Japanese eat Sushi and so it must be a type of Sushi. I think even Wikipedia calls it a sushi, although it's clearly not. But they have weird ways of deciding how to categorize and name stuff there.
And I doubt it's a pun. If it was, it'd probably be a romance comedy or something…it's obviously not, it's just the series is named after the treasure Gold Roger left behind, since when you find that you become the Pirate King.