The difference is that a wagon checks to see if there is a path, and then it is instructed to follow it exactly. If anything changes then it just explodes rather then recalculate. The equivalent of using it's pathing for large creatures would be a dragon that would appear on your map, you would shut your doors, and then it would just die instantly.
A bit late to this thread, still reading through a bit, but I remember one (40d era) occasion where a string of wagons came onto a map, and started heading for my wagon entrance, and then hostiles arrived, from another direction. I shut the main door on the enemies (while the wagons continued onwards, still, at their slow pace), which was a drawbridge across the 3+ width entrance concerned, and was dealing with them by various attacks led out of secondary (non-wagon-friendly) flanking doorways.
The hostiles fled, I kept my military chasing them, and the wagons (unmolested) ended up at the closed wagon-entrance. They all went right up until that entrance, then stopped. On the same 3x3 (plus hauling creatures? ..can't remember what
they did) tiles. I dropped the drawbridge (1xN, set up as a pseudo-wall, IYSWIM) and the wagons continued inwards. All overlapping each other, still, though some were faster than others, I recall, so they 'blurred' back into only an overlapping line (rather than completely overlaying single entity) while they made the short way from this entrance (where they'd bunched) to the depot itself.
However, the code for wagons has likely changed (for one thing, it was withdrawn for an extended time, and I
still haven't had wagons in any new version, for one reason or another, though I'm told they're back, presumably with new code), and a wagon "losing a wheel" but just keeping limping along[1] never happened, but I expect a MTC that's a quadruped with a missing leg could technically be give a MT appearance with one quarter missing.
Yeah. anyway, my opinion... Right now, wouldn't work.
Perhaps, like MTTrees, the wall/block structure could make a MTC of significant size, but unless something like the art decision behind the appearance of 3x3 workshops (with worktops and tools 'representing' the workshop, albeit only ever in one direction) could utilise interesting CodePage-wotsit characters, I can't see anything being made that's effectively 2x2, 3x3, or even 4x4, except the most basic 'lumps of meat', or (like the cart, or indeed siege engines) luckily transposable.
In an
advanced version of the DF interface (includes mutli-level effects through fading, perhaps even
scaling of background characters, I could imagine a 2x2-sized version of a standard golden-yellow "L" tile being an actual 2x2 Giant Lion (or whatever)... Would also work well with non-default tilesets (another problem to look at). Maybe I'll mock up something, before I next post here. Or maybe I'll not have time. Either way, I also have a few pages more of this thread to read, so what I've written may be ninjaed or irrelevant, now. Forgive me, if it is so.
[1] Now I'm humming "Three wheels on my wagon, and I'm just rolling along!".