And for some reason that made me wonder: Is it possible to mod in weapons with absurd lengths? Could you make a weapon with a length of 50? If so, would it have any effect on the game?
Yes. Its effect would be that it could roll repels for pretty much any melee attack against itself or allied units in its square. How effective that would be would depend completely on the unit's stats (and since a unit gets an additional -2 penalty to their repel roll each time they repel in a turn, its effectiveness would be limited).
Repel tends to be a great thing for mass produced units like pikemen since they benefit from being able to repel for eachother, but isn't wildly useful for a single thug or SC.
Additional info on repel mechanics:
If a unit attacks an enemy who has a longer weapon than the attacker, the defender may actually repel the attack and prevent it from happening. This occurs as follows:
Any defender that has a longer weapon than the one used by its attacker makes a repel attack automatically upon being attacked. The attacker makes a defense roll. Every defender in the attacked square gets to make a Repel attempt against this defense roll (using their Attack) if their weapon length is greater than the attacker's weapon length. Every repel attempt adds one to the required weapon length for repelling.
If the result is a hit, the attacker must make a morale check against 10, with modifiers as below:
Attacker: morale + DRN + (attacker size - repeller size)
Defender: 10 + DRN + (the number by which the defender won the Repel attack) / 2
If the attacker fails this roll, he must immediately abort his attack. If the attacker passes the morale check, he may make his attack, but the defender’s attack then generates damage and protection rolls. If damage is inflicted, the attacker takes one point of damage and finishes his attack. In the rare case of a defender with multiple long weapons there are separate rolls for each weapon.
This simulates the defender successfully placing his longer weapon between himself and the attacker. All of this occurs before the attacker’s strike is resolved. Note that units with low morale are more likely to be repelled, and thus using long weapons against low-morale troops is very effective. Also, units with claws and bites (weapon length zero) are easier to repel.
A unit gets -2 to its Repel roll for each time it has been attacked that turn. So it’s easier to repel the first attack than the second, and so on.