I am in the process of building a small off-grid second home and need some advice on attaching siding over foam. My building is constructed with 2x4, 16in. OC framing to which I have sheathed with Huber Zip System 7/16 OSB. I would like to cover the OSB with 2in. of Polyiso foil wrapped insulation and then install polymar/vinyl shingles directly over the insulation using 3in screws thru the insulation to completely penetrate the OSB. I have contacted CertainTeed and they said no foam can be used under their Cedar Impressions siding and another manufacturer that makes a product that specs almost exactly to CertainTeed said go for it, it will be fine. I guess it is a no brainer as to which product to use but I was hoping someone reading this might have some first hand experience or expert advice.

Just make sure you tape all your seams and seal the top and bottom properly.

This comes up every now and again on building blogs. At 2" of foam you're right at the limits of where you can mount vinyl siding through screwed to the sheathing. Most recommend using 1x4 furring through-screwed to the studs with timber screws with at least 1.5" penetration into the studs, and mounting the siding to the furring, and that would be the more reliable/conservative approach for you. See:

http://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/can-vinyl-siding-be-applied-over-furring-strips

http://www.greenbuildingadvisor.com/blogs/dept/musings/all-about-rainscreens

The problem with using 1x4 furring is that the polymer shingles are staggered/randomly fastened unlike a clapboard type product. As some have recommended I could install OSB completely covering the rigid foam but I was hoping that I could save myself the time and expense and just go directly over the foam. Depending on if the nailing flange would allow, could I possibly attach the siding using more fasteners than normally recommended?

Increasing the fastener count would probably work, but unless approved by the manufacturer there could be warranty issues down the line in the event that it doesn't quite work. It's a matter of how much risk you want to take on that end.