When the presiding officer excludes testimony or documentary evidence, the party offering the excluded material shall be permitted to make an offer of proof prior to the close of the hearing. The party may make the offer by dictating into the record or submitting in writing the substance of the proposed testimony or by tendering the documentary evidence for inclusion in the record. Except for cross-examination concerning matters relating to the admissibility of the testimony or documentary evidence, cross-examination on offers of proof shall be deferred until such time, if any, that the testimony is admitted into evidence. The presiding officer may direct that offers of proof be transcribed separately. Failure to make an offer of proof may constitute a waiver of any objection to the exclusion of the testimony or documentary evidence in question.