The construction folks are supposed to reopen the trail each Friday at 4:30 pm, according to http://www.friendsoftheriverfront.org/files/trail_status.html and http://www.post-gazette.com/pg/11067/1130540-100.stm
As far as water levels, Google Earth indicates the lowest part of the Hot Metal to Glenwood segment is the part that's paralleled by Water Street. South of there the trail's around 730 feet, about as high as downtown Pittsburgh, as is Water Street. So my guess is at worst you'd have to use Water Street if there's water on that part of the trail.