***** NOT A TEACHER ****
I cannot answer your question, but I can tell you that, yes, sometimes as is, indeed, a relative pronoun.
Here are some examples from one of my favorite books:
1. Such papers as these (are) must be destroyed.
2. Such of the neighbors as were invited came early.
a. The book tells us to compare it with: Those of the neighbors who were invited came early.
3. Such funds as are needed will be provided.
4. Such a record as this (is) should not be forgotten.
If you ever want an excellent description of all the uses of that "troublemaker" as, try to get a copy of a book entitled English Review Grammar by Walter Kay Smart. You might be able to find a copy in a good library. It is an older book, so I do not know whether it is still in print.
P.S. Another of my favorite books explains that "as" is really a conjunction that is being used "to do the work of [a relative]."
Student or Learner