You don't get anything special for getting all the Dragon Coins in the SNES version. One reason why you can tell is that the game doesn't really keep track of which coins you already got; even if you save, all coins that you have previously collected reappear in the stages after you turn off or reset the game and start playing again. This is not the case in the GBA version; it keeps track of all the stages you get 5 coins in.
There is only one coin in Chocolate Secret in the SNES version, for some reason. There can't be any more, since they wouldn't be practically impossible to find if they did exist. At least two Dragon Coins would certainly be readily visible in a stage which has only one pipe shortcut (which doesn't contain a Dragon Coin) and which, as far as I can remember, has little or no vertical scrolling except in the sliding room (and I think I checked that room thoroughly).
EDIT: For clarification's sake, "Dragon Coins" = "Yoshi Coins". The correct term is "Dragon Coins" according to the SMW manual, even though Yoshi is technically a dinosaur rather than a dragon.