So why not mod a unit that has 1 strat move, 0 battlefield move, no meele attack, a huge siege bonus and a powerful ranged attack with just one ammo, is vulnerable to fire, a mindless construct, immune to poison, a little resistant to cold, has decent hitpoints but no armor? You can have all this in Dom2 if I am not mistaken.
I do not worry about history too much, but I worry about fun. So where is the fun of gameplay in this siege engine proposal? It was fun in the WarlordsII & III to move siege engines around that provided a nice bonus to your overall army, but which was crap when it stood on the frontline. Since they were slow, one would move a siege engine by its own ahead to the next target, so your enemy could sent fast units to intercept it. That was fun. However I cannot see how this would carry over to Domininos: the scale is much larger, and intercepting armies doesnt work in Dom2.
(Hopefully there is a command in Dom3, which gives fast units a chance to defend one or two neighboring provinces to allow some limited interception.)
However, I do see a problem with sieges in Dom2, too: Where is the point for besieged troops to try and break the siege? If they wait they have their towers firing as a helper. The only reason might be starvation, but this is hardly an issue thanks to lab-teleportation of wineskins into sieged castles.
Recovering tax and gem-resources? Hmm, not that urgent, is it?
So maybe every break siege attempt may recover some castle defense points and thus prolong the siege, thus modelling that the sortie of the defenders interrupted the enemies siege. Should be based on the summedup strength of the troops that try to break the siege and the number of battle turns that they manage to survive, so that continually breaking sieges with a single scout won't have any effect at all. But then again it should be good enough to set off the loss of the troops for the next few siege rounds, too. Hmm...