I had been planning out the plumbing system of my Earthship inspired home-to-be, when I came across a venting solution that I hadn't encountered previously:

Air admittance valves

I recognize the cost savings from less plumbing materials, and roof details, but what really got me excited was the prospect of eliminating a roof penetration altogether.

Does anyone have any direct experience with installation of these valves or encountered inflexible codes that would not allow them?

Some jurisdictions allow them; some do not. The only way to know for sure is to read the code in your area.

What I can tell you, though, is that in no case are AAV's allowed to serve as a vent for a system that needs to be able to drain as well as fill, such as:

-Cisterns (some codes require vents even though they are filled from a gutter)
-Sealed collection tanks such as greywater storage
-Water closets (the old fashioned type)

If you really don't want a roof penetration, you can go through a wall, as long as no part of the vent runs horizontal. (My local code says 72 degrees minimum, and the top of the vent will, regardless, have to terminate 6" above the roofline [10" for snowy areas])