I think the *be* in this phrase might be the [lexical be][1], which is mostly used in negative constructions now, but I understand was more widely used in centuries past. 

    In CGEL page 114 has info related to lexical be usage, including examples:

    Lexical be

    This is found with why + do and with if:

    [63]

        i.a. Why don't you be more tolerant?

        i.b. Why doesn't he be more tolerant?

        ii.a. If you don't be quick you'll lose.

        ii.b. If he doesn't be quick he'll lose.

        iii.a. % If you be quick you'll win. -- (grammatical in some dialects only)
        iii.b. * If he be / bes quick he'll win. -- (ungrammatical)


**Edit**: Here is a construction with a positive lexical *be*. I think it's a modern novel that is set in earlier times:

["Be you friend or foe of the crown!" a voice called from a mounted shadow in the distance.][2]

**Edit** I think I thought of another one, a commonly-used, if archaic, phrase: "*Be that as it may [be]*", meaning "as it is".

  [1]: http://ell.stackexchange.com/a/44299/9675
  [2]: https://books.google.com/books?id=DaGpBgAAQBAJ&pg=PA422&lpg=PA422&dq=%22Be%20you%20friend%20or%20foe%22&source=bl&ots=p6f4lmvlSW&sig=TnzPkbjw9L6_oxF-3QzZJ4dJ7Ao&hl=en&sa=X&ved=0ahUKEwjtpLTOo4_KAhWFoD4KHa2PCJYQ6AEILjAD#v=onepage&q=%22Be%20you%20friend%20or%20foe%22&f=false