are phrases like “assume it be that” attested as usages of the subjunctive
@aescling sounds subjunctivey to me
@Lady what i meant was whether this usage is attested
@aescling if the usage is grammatical and you made it then it is attested by you :P
@Lady 
is “assume that” imperative or subjunctive
@aescling "assume that" is just a long form of "assume"; cf. "assume that it be the case that"
@aescling or “assume it be the case that” etc
plain “it be that” operates pretty much entirely to create the subjunctive mood with no trimmings and i think most english speakers today would just drop it if it didn't have anything else to add