Interesting! My column is made up of dry rock with a PVC and foam frame, and I can't say I ever had an uncontrollable HA outbreak. In fact, I only had one in this tank...and although it was a big one, I'm certain that the foam wasn't a contributing factor. It's on page 8 of my tank diary. The reasons I'm sure it had nothing to do with the foam are (1) it happened over a year after the start of the tank, (2) I had some large soft corals turn to goo and die right before the outbreak, adding to nitrate levels, and (3) poor husbandry on my part at the time. Once the cleaning was underway, the HA went away pretty quickly.
I've often thought about the foam projects of ours and HA outbreaks, though. I also have heard some horror stories. Like Flower, I have considered the possibility of silicate leaching into the system. I wonder if people are adding their foam structures too soon. These things need to FULLY cure, which in my opinion takes over a week. Immersion into saltwater too soon would amost certainly stop the curing process, and any uncured product would be much more likely to leach into the water. My stand cure for over 2 months, which is probably overkill but I wasn't in any rush at the time. My rock stands for the 110 tank, by the way, have been re-foamed months ago and probably won't see a drop of saltwater for at least 3 more months. This time around I used the UV-resistant black pond foam...I want to see the difference in breakdown between it and the tan stuff I originally used for the horse tank.
I will say this. Foam has a great benefit -- and simultaneous drawback -- of being as porous as it is. Once HA gets a good hold, it IS hard to fight it off. But not impossible. The first step to preventing it is dilligent maintenance. I think Flower is right when it comes to her root causes of HA, and you see the problems she fought against. My guess is as long as you keep a hawkish eye on the tank, do your weekly water changes, and avoid the miracle mud (sorry, Flower!!), you'll probably be fine.