All of a sudden, now that I am 7 months along, I am having the same issue! It is making me crazy, too. Worse even than just my pants slipping, is that they are pulling down my undergarments too!
Oh, I've been having major pants/underwear disagreements over the last couple of weeks, too. I'm about to start wearing togas, man.
OP, might a long tank as buffer between your poor stretched out scar & pants band help? I've found that gives my pants something to sorta "grab", too, instead of dragging on & irritating my skin.