Not reopened it, but AA has held it open for me when my inbound connection was delayed. A call to the EXP desk took care of it.
Had it been more than a few minutes, I don't know if they would have. Since the chances are more than 100:1 that I'll be one of the folks in the seats the next time I'm near a situation like that, not the guy on the electric cart being rushed to the gate, I kinda hope they wouldn't.