Two points from my perspective. On the couple of occasions that I've launched with the airbrakes unlocked I've happily noticed pretty quickly and got them shut without disaster. But the underlying reason for taking off with brakes unlocked on both occasions was a delay in launching, and a presumption that I'd already checked the brakes when the launch sequence resumed. That's why I now teach Bloggs to open the canopy when there's a launch delay, because "Canopy" precedes Brakes in the pre-flight checks, and the canopy open is a pretty good clue that you need to do brakes again.
Also, if the tuggy notices that the glider has its brakes open (which he probably will, as he's now dragging a large garden shed into the air) he has a couple of ways to signal this to the glider pilot. One is to signal with rudder reversals, but if you're low and slow, the last thing you want is an asymmetric stall, so they may not feel inclined to do this. Of course the other way is to use radio, assuming the glider pilot has remembered to switch the thing on and is on the same frequency. But in the final analysis tuggy may revert to self-preservation and release at his end, and the glider pilot will now be pushing the rope.