I've successfully used LFG certs by calling web support.
As everybody knows, current united.bomb is terrible at displaying green star even if it is supposed to be eligible. Use united.com to hold your reservation, then call web support to book it. They will review the fare rules, and refund the amount of certificate. Last time I did this, it took me around 25 minutes on the phone, but I got refund to my cc within 10 days.