How to keep the fruit on an upside-down cake?
Instead of worrying about how well the pineapple is sticking to the cake, I'd try different pan preparations to make sure the pineapple doesn't fix itself to the pan.

Looking at the recipe that Holly linked to -- it says specifically not to stir the butter and sugar together, which I assume would give you a better separation as there's butter against the pan, and you're doing it in a pre-heated pan, which helps to prevent sticking (but normally isn't done for most other cakes, as it'll cause them to dome on top)

If you're not making the caramel in the same pan (and so you wouldn't have that layer of melted butter), I'd be inclined to use a parchment circle with another layer of fat over it, so after you invert, you'd then have to remove the parchment (which shouldn't be too bad if done quickly, before the caramel sets)