The bolt on the Mod 94 is held in place by two lugs at the rear, which come up into place when the lever is returned to the end of its travel under the butt stock. The only way I know of to fix the headspace on a 94, is to take off the barrel, set the shoulder back one thread, cut off the chamber so that it is the required length longer to recut the chamber so that you have proper headspace.
With rimmed cases, headspace is measured on the thickness of the rim.
If there is another method, please let us know. It has always been a constant irritation for me with lever action rifles.
In the case of the 94, it is possible to fire form the cases by installing an O ring in front of the case rim, chambering it and fire forming the case so the shoulder is advanced enough to keep it tight against the bolt face. Once the cases are fire formed, neck size only and reload. This isn't perfect but there is so much taper on the 30-30 case it doesn't cause any problems.
For straight walled pistol cartridge chambered 94s, there really isn't another fix that I know of, other than my first suggestion.