This is the first I've head of Fe dosing to control phosphates. Some GFOs may be doing this intrinsically, but I've never heard of it being dosed explicitly for this reason.
So - which source for Iron do you use? I'm a wild experimentalist myself so I'm willing to try... I see you use Sera Fe to test.
Anecdotally, I lost a pair of "stainless steel" clippers in my overflow once and when I found it again, it was red and corroded. My tank didn't suffer at all - probably lucky that it didn't have other nasties in there. I always wondered what effect that had.
Maybe the new trend will be dropping a pure iron ingot into the sump?