The Halfswording and Mortschlag rules I suggest are similar to but more restrictive in how blows may be struck than current SCA experimental halfsword rules. They would also allow limited grasping of an opponent’s blade.
Halfsword: A combatant can grasp the blade of his own sword with one hand and use it in all ways as though it were a short spear. He can parry, lever aside his opponent’s weapon and thrust if his sword is equipped with a tip legal for two-handed thrusting. He cannot cut against with the edge of his blade as though it were a glaive.
If an opponent’s blade is being used in this way, you can briefly grasp it if the blade is not in motion. Prolonged wrestling with an opponent’s blade is unrealistic and discouraged. If an opponent would be able to yank an actual blade back through the grip in spite of it being grasped, this could result in an incapacitating wound, and judges may rule that this has occurred.
Mortschlag: A blow struck using either the pommel or cross of the hilt as a striking surface, effectively using it as a mace or hammer. This is only allowed if the hilt meets the standards for a polearm or hafted weapon. The blow was also written as mordtschlag, or called tunrschlag or schlachenden ort (literally, murderstroke, thunderstroke, or battering point.) More simply, medieval manuals also called this blow “striking with the pommel”.
Argument: Halfswording and Mortschlag are well attested medieval combat techniques, and the above rules are both realistic and as safe as other SCA rules for rattan combat. However, grasping the blade and using it to make edge blows against armored opponents does not appear in medieval fighting manuals, and practical experiments with reproduction swords suggest that it is not practical to make efficient edge blows capable of doing damage through armor in with a sword held in this way. A sword blade is a profoundly uncomfortable tool handle for this sort of work. Slices against unarmored flesh are a different matter, and at least one such technique appears in Fiore’s Flos Duellatorum, but it is not relevant to attacks against armor.
Grasping an opponent’s sword is a frequent technique in medieval fighting manuals. Purely from the standpoint of a modern recreation, this is no more unsafe than grasping the haft of a polearm, a tactic allowed in SCA combat. However, since the combat rules are intended to simulate the use of a steel sword that was at least partially sharpened, the grasping rules recognize that you can’t simply hold onto it indefinitely as though it was a rattan shaft. Pre 17th c. combat manuals recognize that if one swordsman is grabbing another’s blade, if the second swordsman is able to yank his blade out through the other’s grip the results are likely to be very unpleasant. Techniques to take advantage of a grabbed blade are usually designed to take quick advantage of a momentary opportunity, and when possible twisting the grasped blade at an angle that makes it difficult for the owner to pull it back sharply.