Does this poncho have a provision to...
Does this poncho have a provision to tuck-up the 'extra' in back when worn WITHOUT a pack?
Yes, the small stuff sack that it comes with it has two cords on it as to be able to store the 'extra amount' but I've always managed to put it back to it's original size by closing the first cord & pushing it hardest I can into the second section of the sack & zips it up.
I do not roll it up or anything like that, I just stuff it in.
Yes. There is a drawcord that cinches up the rear fabric, though for me (6 ft) the poncho does not hit the ground even when I'm not wearing a pack under it.