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If you look at the US Steel site they say very clearly:

Consistent with United States Steel Corporation's ("USS") policy over the last two decades, USS reiterates and reminds that COR-TEN® steel sheet products should not be sold when the intended use is for an architectural application, such as roofing and siding. USS has consistently maintained this position because of the risk of corrosion from factors beyond the control of the COR-TEN® steel licensee (e.g. improper design, fabrication, erection and/or maintenance).

Reasons

* The tight oxide skin of COR-TEN® Steel reforms after abrasion from snow, ice, sand, dirt and hail.
* Acid rain also causes the skin to reform.
* As the skin reforms the product actually becomes thinner and eventually will be perforated.

Application

* Special attention must be paid to the drainage of storm water to prevent staining of surrounding structures, sidewalks, and other surfaces.
* COR-TEN® Steel must be kept free from debris such as leaves, pine needles, etc. These waste products retard the wet/dry cycle necessary for COR-TEN® Steel and corrosion is accelerated."

As an architect, when a manufacturer says don't use something, it is lawsuit city when you do, no matter how nice it is.

I wouldn't use it unless you're OK with the idea that it's a sacrificial aesthetic layer (that is to say, not important to the integrity of the building).

Also, note that in most cases weathering steel does not arrive in weathered form -- it looks like normal steel, and takes a while to form the rusty layer that architects are so fond of. There are ways to accelerate this (ask your local sculptors for advice/references), but in the absence of special treatment you will have a shinier surface for a while.

If you have the money, personally, I would use both the galvanized layer AND the A606 (Type 4, specifically) -- use standoffs to mount the A606 on the outside of the inner galvanized layer (not by much, like 1/4" or so). That way when the A606 fails it's a simple matter to replace the failed sheet, so you can get as thin sheets as you want.

Alternatively, use a thicker form, like A588 sheet, or even a bridge steel like A709 -- that stuff will last practically forever.

...as for the pollution concern, the runoff is primarily iron oxide. Which is everywhere in the environment already; the planet is practically made of it. The amount of chromium involved is microscopic, and is probably less than the amount that's already in the dirt on your site.

Again, if you go with A606 (even Type 4), make sure your details do not allow for any standing water or continuous water contact (eg: no exposure to soil).

Our neighbors used reclaimed steel roofing material as part of the cladding of their house, along with painted vertical shiplap pine (same as we used, although ours was rougher). They wanted it rustier than it already was. and used muriatic acid to help the process along. I'm sorry, but that doesn't seem very green to me. Also, it's a very divisive look - some people love it, and some hate it. (At least it's downhill from us, so any potential iron oxide runoff doesn't go towards our house.)
Rusty exterior pic on Flickr