As you say James, mostly they tighten the gib to lock/clamp the sliding part – although sometimes there is a separate clamp.
The simple way to do it (assuming a straight gib piece) is to substitute a longer screw for one of gib adjustment screws and have some form of handle/lever on it – or for a quick fix – substitute a cap head screw and use an Allan key..
A better way is to keep the existing gibs screws (& lock-nuts) "as is" – drill and tap a new hole and add a locking screw & lever – assuming there is space to do so without weakening the tool. The reason being that you then keep the correct number of gib adjustment screws and don't mess with their adjustment when locking the gib.
Regards,
IanT