Hello mrbradshaw
You are referring to the use of the Swiss Half Fare Card (do not get confused by the Half Fare Card for residents).
Nope. The (Swiss) Half Fare Card validity must cover the entire validity of your peak pass, too. It is not sufficient that it was still valid on the day when you purchase had purchased the pass; alike with train tickets.
Furthermore, you cannot purchase reduced tickets online, you can only get them on-site in Zermatt at the base stations of the cable car or gondolas operator.
Kind regards,
Roland