Honest question here (I didn't play the map on the PTS): Can't you stealth your way up and use knockback on the sniper to force him down ? Entrench is not up 100% of the time, just push and watch him gets eaten by the melee in the middle.
You can but with the Sniper's stealth detect it's tough. Also the Sniper will probably be facing one of the spawns, which would make approaching form the back pretty tough. So with the short KB from Sin's I have seen people get hit with it, myself included, and not travel far enough to get knocked off since you can't hit always hit them perpendicular to the pipe to get them down.
I'm not saying it's impossible but it's a lot harder than it is for the Sniper.