To my ears, the "as" in the first pair sounds unnatural.  In all the other
pairs, both "which" and "as" sound natural and seem correct.

On Fri, 27 Aug 1999, SIRAISI Tomio wrote:

> Dear everyone
> This time, I would like to know the difference between "which" and "as"
> used as the relative pronoun.
> Both can be used when the antecedent is not a noun (phrase) but a
> sentence.
>
> Are both of each pair correct and natural? Or only one of them is natural
> and correct?
> Perhaps, both are wrong?
>
> 1-a. You carried the luggage yourself, which was not necessary.
> 1-b. You carried the luggage yourself, as was not necessary.
> 2-a. He is a foreigner, which is evident from his accent.
> 2-b. He is a foreigner, as is evident from his accent.
> 3-a. Her feet were bare, which was the custom in those days.
> 3-b. Her feet were bare, as was the custom in those days.
> 4-a. She has married again, which was natural.
> 4-b. She has married again, as was natural.
> 5-a. Tom was not at home, which is often the case with him.
> 5-b. Tom was not at home, as is often the case with him.
>
> Would you tell me your intuition (and why).
> Thank you in advance.
>
> SIRAISI Tomio
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>