It's easier to deal with the second example first.
It isn't for such as them.
You may note that I've dropped the infinitive phrase. That should have no impact on how you analyze the word in question.
This "them" is an object. Specifically, it's the object of the preposition "as". We use the same case for the objects of prepositions as we do for the direct and indirect objects of verbs.
There is an obvioius way to have the subjective case make sense. Use the word as the subject of a verb. One such example is:
It isn't for such as they are.
In this case, "they are" is a clause. The subject is "they" and the verb is "are". We can still treat "as" as a preposition, but, if we do, we have to realize that it's the entire clause "they are" which is its object. Clauses don't change case. It's also common to call this "as" a subordinating conjunction.
The verb in the subordinate clause is not a repetition of the verb in the matrix clause, although both verbs are forms of "to be".
none [are] so blind as them
none [are] so blind as they [are [blind]]
This example offers you a choice.
The simplest choice is to treat this the same as the first. The "them" is the object of the preposition "as". On its own and as an object, it takes the objective case.
It's also possible to treat the pronoun as if it were not on its own. In this construction we can repeat the verb and adjective complement of the matrix clause to form a subordinate clause. Since the clauses are parallel, we can even leave the predicate unwritten in the subordinate clause.
You may note that I've dropped the relative clause which modified "they/them". Like dropping the infinitive, this should have no impact on how you analyze the word in question.