Anthony,
You bring up some really good points, so let me see if I can address some of them.
When I say "slightly larger," I don't think it's probably as bad as you imagine. The concrete pad is only three inches wider than the building footprint.
I did this specifically to allow for 1-1/2" of sheathing, siding, and trim so the finished building will be exactly 10' x 10', and thus no nosy neighbors can go whining to the HOA.
Now, given that the LP siding will be 3/8", I may have actually a 1/8" extension of pad beyond the finished edge of the wall.
The sill plates will be made of PT lumber, and I intend to either roll styro insulation between the pad and the bottom of the plates and/or seal the outside seam between the plate and the pad. I also intend to apply some of that black fabric material along and extending slighly below the bottom edge of the framed wall before the sheathing is applied.
What I think I will do is to actually run a "ring" of cedar trim around the base of the building all the way around, covering that troublesome latter 6", then seal the joints around all the trim. The siding is all pre-primed, so I'll prime the trim and give the entire building at least one coat of a good exterior latex.
As I think through it, I believe that will probably do as much as can realistically be accomplished short of having an excavated footing that rises from the grade, and that would have been a project more expensive than the building itself...
Hope that makes some degree of sense...
Ongoing thanks to everyone for their continued help and suggestions.
-> Intrepid,