The various style guides that I'm familiar don't alter their guidelines for title capitalization depending on whether an included preposition (such as in) is part of an idiom or not.
That's not to say that none of them ever alter their preferences in special instances—such as when the preposition is more closely associated with the verb than with the associated prepositional phrase, as discussed in earlier Q&A's on this site (How to capitalize "get out of" in a title according to CMOS?How to capitalize "get out of" in a title according to CMOS? and http://english.stackexchange.com/questions/187824/capitalising-for-depending-on-the-usage-in-the-title#comment392421_187824https://english.stackexchange.com/questions/187824/capitalising-for-depending-on-the-usage-in-the-title#comment392421_187824).
Style guides do the darnedest things, so I wouldn't be stunned to learn that some guide somewhere does treat prepositions in idiomatic phrases differently from other prepositions playing the same role but not in idiomatic phrases. From a reader's perspective, though, I don't see any advantage to be gained from making such a distinction, which is probably why most style guides do not endorse treating prepositions in idioms as a special case.
