Taking Maarten’s sentence as a starting point “When 'on' can be replaced by the specific subject pronoun 'nous', adjectives agree with number and are therefore plural (only the past participle/adjective, not the auxiliary verb conjugation).”
My question is, assuming that we have decided to indeed follow the agreement rule, if the specific group that “on” refers to were all female, would the sentence then be “On était éberluées”? I.e. does the gender get reflected in the adjective in the same way that it would if we used “nous” and the group were all female (“nous étions éberluées”).