Guys, I have successfully modded siege weapons to be able to be build in the field by soldier! Behavior isn't 100% satisfactory, but is 95% of the way to what I think want to be. The only thing is that the Onagers swivel (this is right word?) too fast. I think they should swivel slower to face their target. Other than that, my new Field Artillery tech work wonderful!
Right now, the tech, Field Artillery, is only enabled for the Principate Romans. I will enable them for other catapult civs eventually. Research Ballistics at Fortress. This both unlock ranged siege weapons and unlock Field Artillery tech at Fortress. Research Field Artillery to allow Principate Roman infantry soldiers to build stationary Onagers in Neutral, and Enemy territory. So very good to siege cities! I think I will make stationary Onager catapult the field artillery for the Principate Romans, and then allow Heavy Ballista, a packable, moving siege weapon, for building at Fortress. Right now the field Onager cost more and take longer to build than Onager at Fortress. They also still cost housing.
Another change I make is to have the Flaming Munitions tech swap actor of the Onager to make the stone go to flaming only after researching the tech. This reminds me of Age of Mythology how they did this and make a visual understanding of the effects.
Guys this make me happy that these thing is achievable with the current engine.