I think the be in this phrase might be the lexical belexical be, 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.
Edit I think I thought of another one, a commonly-used, if archaic, phrase: "Be that as it may [be]", meaning "as it is", or, "with things being what they are".