I’ve flown non-rev for years (my brother works for an airline). I ALWAYS wear long pants (usually slacks, or even coat and tie).
When my son was a captain for a Delta affiliated regional airline, I flew a number of times between Seattle and Houston. Always wore slacks, polo shirt, and dressy casual shoes.
Met my wife on line. Flew down to Houston from the Oacific Northwest about 20 times over a year before moving down to Houston. Spent 4-5 times overnight at MSP. If you have to spend a night at an airport, you can do much worse than MSP.
First, I appreciated being able to fly for free. Second, there were a few occasions when I was seated in first class. If you are not dressed nicely as a non-rev, you aren’t ever going to be seated in first class.
My son is a pilot for the Big Delta. Don’t fly non-rev anymore because getting to and out of PDX or SEA is very iffy. Now that he flies for the Big Delta, my priority is higher, so I might give it a go in the next year or so.