FAA-legal, yes. For example, Meigs was closed at night, but the FAA never got involved on the numerous occasions pilots used it after dark-- that was strictly a City of Chicago thing. Cops would actually write tickets to pilots that landed there when it was closed.
I've personally departed on closed runways twice: first time at FNT when all three runways were closed for various reasons. I knew that 9/27 was closed solely due to construction on the approach end of 27, so I departed 27 from an intersection (in an Cherokee 180) after the required back-and-forth with the tower as quoted above. No repercussions.
Second time was at Palwaukee when the airport was closed due to flooding. The first 2000 feet of runway 16 was underwater (and expected to be that way for days), and LOTS of us took off on the remaining runway, just to get off the airport. Tower even put the "takeoff will be at your own risk" phraseology on the ATIS. No repercussions.
If either of those situations had gone sour, I have no doubt that there would have been repercussions, in the form of a "careless and reckless" charge-- but most FAA folks seem to go by "no harm, no foul". They rarely pursue an isolated incident, even if it shows questionable judgement, if it turns out to be uneventful.