I'm not planning to do a bog type anoxic filter, but rather one that attempts to achieve anoxic conditions within the core of solid media.
Iron bearing lava rock in large chunks probably work, as do Dr. Novak's clay granule and fluorite red baskets.
Sulfate/sulfite generation happens when the system tilts towards the anaerobic end of the equation. Under high flow conditions where the media is surrounded by moving media on all sides - including underneath it (due to suspension off the floor) there should be little chance for H2S generation to occur unless there is a very prolonged system shutdown.
My main concern is the long start up time, and sensitivity of these systems to common medication or treatment regimes. The way I have designed my filter, I am giving the anoxic stage first dibs on any ammonia in solution. This also means I can't isolate that part of the filter. As mentioned in a past post, it it ends up working, all's well. If not, I have my traditional bakki and MBBR to fall back on.
Speaking of the MBBR. I have read that under the right conditions, an MBBR will actually host anoxic bacteria. This usually involves using coarse air bubblers and low volume air to mix a large quantity of media, combined with a relatively high dwell time. I might just go ahead and order 300 pounds of plastic bio chippery in the end If I do that I will have to do something special with the return water from the MBBR tank. Can't be hitting the pond inhabitants with a constant stream of deoxygenated water...
Finally, I'm still scratching my head as to where to source the anoxic seed media. I might end up taking a clay core sample of the local reservoir bottom (in the dead of the night to avoid the rangers).