Just have a shell-only refuse pile indoors and...you know...not use them? Non-moody Dwarves won't go use up your shells unless you tell them to do so. So you don't have to do all that micromanagement to make sure you have shells in stock: just refrain from spamming "decorate with shell" and they won't go anywhere.
Edit: aaaaaand now I see that Wolfius said pretty much the same thing
"all that micromanagement"? what game are you people playing? it's a trivial thing to do, it takes 5 seconds.
There's enough micromanagement as it is already. I just don't see the point of doing something for the sole and solitary purpose of adding difficulty for no reason whatsoever. And that "takes 5 seconds" you mention? Me and Wolfius's way takes literally zero seconds, and doesn't need to be repeated every couple of seasons.
Not to mention the fact that it's a lot easier to notice when the shell stockpile is getting low than it is to constantly check, "how many turtles do I have? Do I need more? Do any dwarves like turtles, and if so, should I unforbid some so that they can eat them? What will I do with the shells, since it's so abhorrent an idea to let them sit there until a moody dwarf needs them?"
Your method doesn't take zero seconds. You have to keep an eye on the shell stockpile. Monitoring a stockpile requires time and attention, AND you can mess it up if you forget to look in a couple season. My way, you literally never have to pay attention to shells EXCEPT when a dwarf needs them. And I've played fortresses where no mood demanded a shell for like, 5 years straight. So it's not 5 seconds every few season, it's 5 seconds maybe once few YEARS. And there's no real way to mess it up.
You only unforbid turtles if someone actually asks for one in a mood. Other than that, you never look at turtles, you never check your quantity of turtles, etc.
Not to mention if you're actually fishing or importing enough turtles, you simply keep 10-20 turtles forbidden in the event that a mood happens to coincide with a time in which no caravan has brought turtles recently, or, if you have fisherdwarves, that a fisherdwarf hasn't caught a turtle recently. So you rarely, if ever, have to actually unforbid anything.
And finally, my method doesn't even require an indoor refuse stockpile. That's an entire building/area you have to set aside to save the turtle shells. Forbidden turtles can just sit in your normal food stockpile, in one or two barrels.