Before loading into a battle, changing memory machine code at address "rome2.dll+6143E1" from 74 64 to EB 11 can force all soldiers to carry a torch. You can try to witness it yourself.
(I have to remind you that many in-game actions are accompanied by memory code check of pre-determined memory sections. If a specific action detects it, the game will crash.)
They do everything as normal except that a torch is attached to their right hand, which can look hilarious sometimes. Their ordinary weapons are either replaced or invisible.
For example, when a unit is in pike phalanx, their pikes are invisible, only a torch at right hand, but enemies still get stabbed 5 meter in front of them, just like they normally would.
The main problem is that game can be laggy with large number of soldiers, light sources, and smoke. The illumination from multiple torches also makes soldiers a little too brightened for my taste.
To see the effect with large army, I bring 6400 oathsworns and prevent them from turning into 2D sprites so that they always carry torches. I also force all torch light to appear regardless of their distance from camera. This is how it looks like: