I've had Phosgard in a TLF reactor on my tank for about a year, so far no problems with zoas. You may want to post a question in the Seachem thread ( the are a sponsor of the club) or shoot them a PM).
I use it because I can pick it up close by( they order it for me) and when I started using it there was some debate on PO4 media releasing some PO4 back it the system, from what I understand Phosagard won't do that. I think that it is aluminum based as opposed to iron, ferric or ferrous based as in GFO
In my tank I've had ALK issues, nearby corals attack and bryopsis problems, but I don't think that Phosgard caused any of that ( alright maybe not changing the media may have caused contributed to the bryopsis ;-).