Several different points:

First of all, the general point is that reducing a (standard, western) human to "zero welfare" would involve inflicting great pain and suffering upon them, which would get you reviled and imprisoned, with judges unlikely to be impressed by your philosophical justification.

I find that a rather odd statement. Isn't "zero welfare", by definition, the amount of welfare such that any life with greater welfare is a life which it is good to create?

That's a bit question begging, as some ethical systems have zero welfare, but don't generally advocate creating people at that level. My favourite definition, for what it's worth, would be that "lives below zero welfare are not worth creating in any circumstances (unless as instrumental goals for something else)".

Similarly, if creating a just-barely-worth-living life is repugnant, then creating any life is repugnant.

Most ethical system that reject the repugnant conclusion also reject that argument. How do they do it? Generally by making the decision on the creation of lives dependent on the existence of other lives. Average utilitarianism would advocate against creating lives below the average, advocate for creating lives above the average (indeed average utilitrianism, uniquely among population ethics as far as I can tell, does not need a "zero" level). Egalitarianism would advocate creating any life above zero that didn't decrease equality, etc...

So all these systems would have some situation in which creating a life just above zero would be good, and (many) situations in which it would be bad.