bartender,
most towbars (and all large aircraft bars such as B747) have two types of shear pins. axial which shear when the turn pressure is too great, and torque which shear when the straight line push or pull is too great.
so if you push when the brakes are on the torque should shear before damage to the gear. not an expert in towbarless tugs, but somebody from douglas, goldhoffer etc must lurk on here.