Unless the wiki's wrong (as well as my experience being incomplete) all hollow parts are 2*2, so this should be safe. How high the hollow part reaches varies individually per spire. I haven't heard of a hollow section above a solid one before, and if it does (still) happen that should be a bug if it's dangerous (since there's no path from the source to the upper section).
The recommended way to check the danger is to carve a fortification in the spire (after making appropriate arrangements to block dwarven exposure to dust/fire/... from the fortification if it's unsafe). There are two big costs for that though, when the safety answer is "no": FPS tanking due to the additional unit pathing, and a significant risk large parts of the spawnees fail to path through to your "proper" breach later on.
I tend to mine everything that's safe first, build appropriate countermeasures, take a deep breath, and then breach close to the base of the spire. It is, of course, a great anticlimax if it turns out to be solid there as well, though, but it sure starts a flurry of mining (potentially resulting in SeymoreGlass' surprise).
Actually, come to think of it, I've encountered one case where there was a hollow section a bit up, but it's a rare exception. It was a reclamation fortress I was allowed to use a save from where the fortress had punctured the spire and the results were camping in the fortress, but the stuff below was blocked off and hidden (I don't remember if the spire was solid or if there was a floor blocking it, though).
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Topic: Erm what? Shouldn't this be more !!FUN!!? (Read 1719 times)