I'm not sure if anyone has, but it's a good idea anyway, and I'm gonna run with it.
The wood interests me as much as the fruit, due to the possibility of using it for things like fine furniture, and maybe incense?
The citrus fruit can be used as a spice (as in citrus zest, or just lemon juice), while the applewood can be used in barbeque, to "spice" meat, as well as to possibly be fashioned into kegs, which would then "spice" liquor.
Lemon juice is also a very good cleanser. Oranges can be turned into marmelade. Blood oranges into Sangria (well, a *type* of sangria, and my favorite alchoholic drink).
If included, lime leaves (kaffir, typically) would be a spice.
These types of fruits-including apples-are excellent sources of Vitamin C, and (along with limes, onions, and raw meat/fish) would help prevent scurvy, if we ever get that.
Not to mention, there's a hell of a lot you can do with apples, food-wise, and even reaction-wise. Everything from cider/liquor (and syrup), to apple-butter, to fruit leather, to using the apples themselves to "spice" meats. The seeds themselves even contain cyanide, so it might be possible for an alchemist to refine that into a poison.
Coconuts are a special case. I'm sure there's lots one could do with them, with coconut oil and etc.
Ofcourse, they serve as both a food and liquid source (the liquid is, unfortunately, a natural laxative, but it's also fermentable, I believe), as well as a ready-made source of bowls/cups, etc.
Not absolutely sure about the quality of the wood, though.
Candlenuts are another idea. They could serve as not only a source of oil, and food I think...but a direct, immediate source of illumination (and even of telling time, combined with rope--made, say, from coconut fibre?).
There could be a chance for dwarfs, etc. to get struck by a falling fruit, while standing under a tree, and fallen fruit could potentially ferment, making animals that eat it drunk.
Something else that would be great would be if certain trees could be harvested for shillelaghs. So instead of fruit (thorn bushes), or maybe instead of a log (thorn trees), you'd get a weapon. Ofcourse, the weapon might have to be further treated (heat-treated, lacquered/shellaced, what have you), but it could give your Fortress defense a boost, and it could be made rare, and uncroppable, for balance reasons.
Speaking of thorn-bushes, I'd like it if fairies would spawn in something like this, like say in the right biome, when a thorn bush or a thorn tree reaches a certain age, it could be inhabited by a fairy-who could then become a "fairy king/queen"-and from that point on, fairies could start spawning from that bush/tree.
Umbrellas/parasols might be fun, and I'd like to see them occasionally weaponized.