Can't say for your flights, but I have a flight coming up from New York to Belize, and there was no difference in fare for an evening flight out of NYC to Houston (and not the last flight of the evening, at least four earlier flights are also the same price) and the connection is not until the next morning to Belize. There was no fare difference between that and flights that did not overnight, or the last flight from Newark to IAH.

So yes, some airlines apparently still allow it, but can't say if US Air will be one of them.