I personally think that this unit would be a bit useless. Here is why:
1. 0 damage times 50000000000 is still 0.
2. If he costs anything more than 1, why not just build an assassin for so much more capability?
3. This could get very out of hand with people zerging you with wagons of these guys, who would one hit all of your fortifications. I mean, alright, maybe diggers did exist irl, but this is very overpowered and deffinately not how it worked. Sieges like this involved years, not wagon zerging.
I think this might have some merit, but there are a lot of flaws to the concept that would need to be sorted out.