- Far be it from me to criticize your work.( I certainly would not criticize your work.)
We are dealing here about the present subjunctive form of the verb that is almost like bare infinitive ( root verb without " to" i.e. go, be, read, play etc.). Here we don't use third person singular "s" and don't require "do" support to form negative. Its use is restricted to formal English, hypothetical situation, after some verbs of order, wish, suggestion and certainty and in some set expression like ' god save the king, ' be that as it may.'
You can easily avoid using present subjunctives with the less formal should + infinitive:( it doesn't work in your example)
I begin with an a example to mean that your example makes use of BE to infuse a sense of certainty, the ulterior motive of being yeomen.