First, let's NOT discuss the multi-threading argument YET AGAIN, so let's just drop that right now. If you wanna derail that, there's hundreds other threads dedicated to it.
You can pretty much mod out anything you don't like. That's neat for the player, gets to tailor the game, but it's gonna get beyond the point of rationality. If you've modded your game to provide free infinite giant ax blades, created from 1 bar, sold to a trader who is brainwashed to buy them for top dollar to protect his home against enemies who's entire sieging tactic is to sit in the dirt... Well, if you're modding that far and playing it as a "normal fort" then you're fine, as long as you never try to talk about it because the community at large will laugh you straight off the internet.
Oh, and ore isn't solid. You don't find iron ore and find a solid block of iron forming the entire wall. Current gold mining is actually pretty much dust, dirt colored dust, no less. The process of smelting is to remove the impurities and distill it down to a bar. It just so happens that the entire wall is an impurity and the dwarves burn away the dirt to reveal a single nugget of copper.
For the record though, tunneling enemies will probably be heavily bugged and hyper-aggressive when introduced, so expect your maps to be considerably lighter after a month of attack. I'd probably have to mod them out if they were acting overly disruptive. I love the idea of siege ladders though, I'd probably leave that in even if they were bugged to HFS and back.