Can this be the only place onteh internetz that discusses this point? There must be a name for the construction.

I imagine the "of" is added because the form is so rare where the article follows the adjective. Posh Brits use it all the time though, so there must be a name for it.

"I hadn't prepared for so late a night"

Is it when you want to modify the adjective then?

"It's not a that big deal" X
"It's a not that big deal" X
"It's not that big a deal" Tick

Sigh, ranting into the internetz instead of doing work is not going to get me out of the office any sooner...