grammaticality - Adding a possessive to a singular noun phrase that ends in a plural noun

Which of these sentences is correct: "The clock under the curtains' hour hand broke off", or "The clock under the curtains's hour hand broke off"? The actual thing being made possessive, "The clock under the curtains," is singular, suggesting that you should add 's to the entire phrase to make it possessive. This would make the latter sentence correct, but it looks funny to me. Of course you don't add 's to a plural noun already ending in "s" in order to make it possessive, but what do you do if the possessive noun phrase itself is singular but it ends in a plural noun?

Answer

I'm going to quote a comment that I think efficiently lays out some of the presuppositions that this question is based on, in order to express some disagreement with those presuppositions:

in principle one of these two options should be grammatically correct (albeit awkward), right? Which one?

It is not acceptable to add the -'s genitive (or "Saxon genitive") to all noun phrases, and in the contexts where it is acceptable, there is not always only one "correct" form. So I don't think it's right to assume that this is a binary question.

The paper The English “Group Genitive” is a Special Clitic", by Stephen R. Anderson, gives some examples of (single word) noun phrases where it is fairly clearly not acceptable to add -'s or -'.

(20) a. *These’s illustrations are more competently drawn than those’s.

b. *Of the books I lent you, two’s/some’s/many’s covers were soiled when you brought them back.

I think these examples establish that it is not impossible in principle for there to be no acceptable way of forming a -'s genitive from a particular noun phrase.

To address the specific noun phrase given in the question, I think that most speakers who tolerate the use of the -'s genitive with the noun phrase "The clock under the curtains" would pronounce the genitive construction no differently from the original noun phrase, which would support the spelling "The clock under the curtains' hour hand." Anderson brings up the topic of noun phrases ending in a word suffixed with /z/ that is not the head of the noun phrase: he says that Zwicky (1987) describes these as not taking an additional [z] sound, but Carstairs (1987) "claims that the sentences with two /z/s are often acceptable".

I would agree with tchrist's advice to "write what you say", if you use this construction at all (my preference would be to avoid using it). But overall, this is a rare construction, linguists don't give uniform descriptions of the usage, and it's simple enough to rephrase in formal contexts, so I don't see any point to being dogmatically prescriptive about there being a single "correct" usage in this context.