As far as I know (and you know, J Alex, that ain't much) they don't permit new daytime only operations…
But however comma, I just came across a CP the other day for a station, but with no nighttime operation. And Radio-Locator seems to list it that way, too.
That station is WPNN, Pensacola. It currently operates 1 kW days from a tower site right in P'cola, with a night site shared with WEBY south of Milton. Both day and night are non-directional. The CP has it upgrading to 3 kW days from the WEBY site, using all three towers there. But there is no accompanying nighttime CP to go with it. So I dunno. ???
Maybe the right thing to do with some of these day only operations is to just suck it up and turn in the license. AM is struggling enough as it is, no nighttime authorization only hampers things even more.