Yes it's possible without annimations :

The number to change is here :
stat_pri 13,4, javelin, 45, 2, thrown, blade, piercing, spear, 25, 1
Infact this number means how many miliseconds the unit will wait between attacks before playing the annimation. Obviously you can't see real difference in combat but a unit with 0 is five time more effective than a unit with 25 this means this can create a strange unbalancing, I discovered this by accident when 120 archers with short swords destroyed and routed a unit of 200 spearmen.

But let's see the good aspect of this :
The more this value is high for everyunits (my mod uses 25) the more are the melee combat slow, without any changes on morale or and defense/attack.
It's a good tool to simulate the fact that units with heavy weapons attack slower adding +5 the 25 would make them attack (or playing thier annimation) slower than others you can use it too for men with two weapons by allowing them to attack twice as fast or slower wich makes them deadly effective without raising thier attack value.

I hope this could be usefull for actual RTW modders and that this value is still here in MTWII.

I've not extensibly tested it but it's makes a real difference, and could be quite usefull for realism mod for exemple but remember this have to be applied in the unit cost to keep things balanced.