Is nirvana a conceptual construction? Is nirvana a conceptual construction - empty in that way? For any / only some Buddhists.
I'm just trying to figure out how extinction can avoid the extremes of eternalism and annihilation. If it is a conceptual construction in the sense of having no reality outside what it is in conventional designation (blissful etc.), that might be an answer. If some rock has no qualities that we cannot define, then maybe there is nothing to add to its existence, nothing to puzzle over and ask why or how.
&quot;I'm just trying to figure out how extinction can avoid the extremes of eternalism and annihilation.&quot;
And therein lies your problem.  If you are trying to figure it out, you are dealing in concepts, not nirvana.  Every word you speak answers itself.  It is a closed loop.  A closed system.  Step outside.  How would you define a word without using language?  See the following:

Case 5 of the Mumonkan Kyogen's &quot;Man Up a Tree&quot;
The Case:
Kyõgen Oshõ said, &quot;It is like a man up in a tree hanging from a branch
with his mouth; his hands grasp no bough, his feet rest on no limb.
Someone appears under the tree and asks him, 'What is the meaning of
Bodhidharma's coming from the West?' If he does not answer, he fails
to respond to the question. If he does answer, he will lose his life.
What would you do in such a situation?&quot;
Mumon's Comment:
Even if your eloquence flows like a river, it is of no avail. Though
you can expound the whole of Buddhist literature, it is of no use. If
you solve this problem, you will give life to the way that has been
dead until this moment and destroy the way that has been alive up to
now. Otherwise you must wait for Maitreya Buddha and ask him.
Mumon's Verse:
Kyõgen is truly thoughtless; His vice and poison are endless. He stops
up the mouths of the monks,  And devil's eyes sprout from their
bodies.

Instead of worrying about concepts, search for the word that has neither sound nor meaning.