(1) I have checked my grammar books, dictionaries, and the Web.
(2) I have no definitive answer to offer.
(3) I, personally, have an idea. It may or may not be correct.
(4) Of course, it depends on how you use that phrase.
(5) But it is often used in this context:
Suzie: Where is Dad?
Tony: Oh, he's reading the newspaper, as usual.
(a) I believe (that) that sentence is short for:
Oh, he's reading the newspaper, as is usual.
(i) I believe that as in that context = which fact [fact =
the act of his reading the newspaper].
(b) Conclusion: "as" in that sentence is a relative pronoun
(that is, it refers to "he's reading the newspaper").
(6) The closest authoritative answer that I could find to back up
my idea comes from the most revered Mr. Henry Fowler (A Dictionary
of Modern American Usage):
Epeiros, as is well known, was anciently inhabited by ....
Mr. Fowler says that "as" in that use is a relative pronoun that
= which fact.
*****NOT A TEACHER *****
Student or Learner