I've been looking at the various window bucking alternatives, and can't see how *any* of them provide an effective air seal between the concrete and the buck.  Seems to me that given the relatively low odds that the concrete is going to form a continous seal to the buck, especially along the bottom edge, plus the lack of a seal to the foam (which itself has unsealed gaps in it at each block boundary), plus the fact that the expansion rates of the concrete and the buck material are going to be very different, the net result is going to be pretty much a direct path from the outside to the inside around the edge of the buck ranging from mm to cm in size.

Seems to me the only effective solution to this would be to remove the buck after the concrete cures, squirt foam around the hole, and then reinstall the buck.  Or is there some other trick I've missed?

Look at V-buck they will answer the questions you have about sealing to the foam.

Did you read the thread from a few weeks ago where Dave (walltech) discusses using 2 x 6 inside the form? That should be a pretty air tight configuration. It's what I'm going to do for windows.

For doors I'll be doing bucks the way you raise the issue about. I plan to use 2 x 12 DF with sill seal foam stapled to the backside of the 2x. I expect the foam to be somewhat compressed by the fresh concrete, and then expand as the concrete shrinks away from the buck. Plus, on the outside I will be placing P & S flashing to form a water barrier over the buck/foam interface. The flashing will block air flow. On the inside the drywall will be fastened to the buck.

I really don't think the air leakage problem will be as serious as you are concerned it may be. But it can be if care isn't taken during construction.

Air leakage will also be minimized by using a good quality peel-n-stick around the windows. Grace makes a good peel-n-stick.
I've used a small amount of v-buck a few years ago. Never had to finish it though. I am not sure how user friendly v-buck is for the finish carpenter.

Bunt

You can dado the back of your wood buck with a 1 inch dado router or wobble blade. This provides a keyway for concrete to enter also reducing air infiltration, also a relief so the wood will not be as apt to cup or twist.

Kevin

I probably shouldn't be promoting v-buck but I have found it to be very user freindly for finishing.

The trim will attach at the extension jamb and unless you have some extremely wide trim it should still be able to attach with a finish nailer at the outer edge of the v-Buck.

In the case of the wide trim some construction adhesive may be used, keep in mind trim is rarely a load bearing surface so it takes very little to hold it in place.

I like to seal 2x12 to the ICF using Tuck Tape then once the windows are installed use Tuck Tape again to seal the window to the wall. Seems to work for me.