Have you tried contacting the manufacturer? They offer chat, Skype and a toll-free number on their website.
Actually, I did chat with ProClip before I posted my question here. They said the phones/holders were not compatible. That said, I wanted to see if any forum members had first-hand experience. Looks like the specs are close enough that it might work ok. I have my 920 holder hard-wired with USB charging.
It's possible that there aren't a lot of people who use them.