When to use this construction: so Adjective a Noun

I don't know if it has a formal name, but it's used not infrequently in more formal writing.

• But his hand fell to his side as though he could not bear the exertion of even so small a gesture.
• This is not so grand a gesture on my part. It is mostly an admission of defeat.
• They said that it stood for “able,” so strong a woman was Hester Prynne.
• The Master, weary from his own shower of blows, and fearing nothing from so weak a man, dropped his hand for an instant, and at that instant...
• As such, their lifespans are ridiculously long for so tiny a creature: up to 30 years (if that doesn’t sound high to you, consider that British men in the Middle Ages had the same life expectancy, and probably did the same amount of toiling in mud). (of a naked mole rat, Wired, 9/12/14)

It's not an unusual construction, and carries a bit more impact than "such a _ _" because of its less common construction.

The structure so + adjective is normally only found used predicatively, where it appears as the complement of a verb:

• The music was so beautiful.

Here we see so beautiful appearing as the complement of the verb BE. We cannot usually use this structure attributively to modify a noun after the indefinite article, a, or when there is zero article:

• It was so beautiful music. * (wrong)
• It was a so beautiful song. * (wrong)

In these instances we need such instead of so:

• It was such beautiful music.
• It was such a beautiful song.

Notice that while so modifies the adjective, such modifies the whole noun phrase. We could just as easily use no adjective at all:

• It was such a song!

Such then can be regarded as a strange kind of word (perhaps determiner, perhaps adverb, perhaps adjective), which modifies the whole Noun Phrase, including the article. Notice too that we say Such a beautiful song and not a such beautiful song. We cannot use such to grammatically modify an adjective in that way. Such definitely modifies noun phrases not adjectives.

However, an alternative way of achieving the same kind of effect as either of methods above is to use [so + adjective] before the indefinite article:

• It was so beautiful a song ...

Here, in this position before the indefinite article, we can use the adverb so with an adjective. Here it is modifying the adjective beautiful. This time the adjective itself, very unusually, modifies the whole Noun Phrase including the article. This gives it quite a literary effect. It is not what we would regard as everyday, informal, conversational English although it is common in descriptive writing.