Sincerely, I haven't any modeling experience, but making windlance as model and making of animations which corresponds to it is probably hard work.
I also had this idea some time ago on my mind, but probably because of some "layman's instinct" about modeling, I have abadon idea of posting it...
No matter of that, I will try to give my feedback on this topic.
For start, that windlance really looks amazing.
I agree with you about concept of dwarven windlance, it is dwarven (impoved balista concept) contrivance oriented for air defence (and it could be used to shoot on monster or untits on the ground from some high position). So I presume that main idea and concept in movie was as anti dragon weapon. I find that concept pretty cool, and maybe we could suggest it in some smart way for this mod.
Concening your suggestion that it is just upgrade for Lake Town, well, I don't see reason not to be in Dale also?
And concering Catapult upgrade in Ered Mithrin, I have always found that one as inappropriate uprage for dragon slayers. Simply dragon slayers are using some piersing objects for slaying those monsters, not some flaming rocks. It is matter of consistency in my opinion.
Idea about hero controlling windlace sound interesting, but I am more for windlance model which has integrated unit which controls it.
I would suggest that as anti dragon and anti air mechanisam with limited number of shots (something like in movie) , and after that some greater CD.
Upgrade to be oriented to air (not ground, or both...) and be upgrade which will do very nice damage against air untis (Dragons, Smaug, Eagles, Fellbeast etc), but limited shoots.
About implementation in game, concerning Ered Mithrin, I was thinking that new Dwarven windlance should just replace catapult on its place, on top of building.
It is similar stuff, Catapult have one shot with aoe effect and great cd, windlance could have few single target shots with also great cd after last in row.
Concering dame upgrade look, it could be visual upgrade for watch tower.
So on watch tower:
http://postimg.org/image/94mm31an7/full/to install this part from movie when we research that upgrade:
and on the top of it, new dwarven windlance:
Similar upgrade (oriented in architecture) for Dale.
Those were some "rough" thoughts of mine.
Regards,
CragLord