I don't think you even need to mod them. They do come out from the siege engineer after all.

I did once try to get them to break down a gate (at a wooden stockade type fort) but even two units together were only good enough to cause maye 50-60% damage by the time they ran out of ammo and I had to bash my way in using other types of troops. They would have been better employed to throw bombs in through the gateway and blow up defenders who bunched together too much.

What I will say for them is that, being such a tiny unit, the arrow fire off the walls wasn't accurate enough to score many hits. I did lose some but nowhere near as many as bigger units tend to suffer if left standing still within range.

For some reason, I still picture the Naptha weapon as being more incendiary, like a petrol bomb, than explosive and hence not good for blowing holes through walls, especially masonry.

It would be demanding on the graphics power but it would be kind of cool if using them in woods would (unintentionally?) start a fire that set both sides' troops fleeing the scene, or using them in arid lands started a wildfire spreading across the grass, to the same effect, smoke affecting your view and so on.