At Air Canada, we used to use a nose down-lock pin to secure the nose gear from retracting while towing/push-back, however, this is no longer used for ramp operations. They used to be used on most aircraft types, however, the only a/c we use it on now is for towing operations with BAe-146's. I beleive maintenance still uses this pin during maintenance operations.
We do use a steering by-pass pin for all operations on all a/c (except B-737's which don't use any pins and BAe-146's).
As mentioned, this pin disconnects the hydraulics so the nose wheel can only be turned by the tow tractor and not with the steering tiller or rudder pedals.