When I did a lot a tilt-up buildings and diaphragms wouldn't work in an upgrade project, we often times used an overlay of sheathing to the roof or floor diaphragm. These were considered effective even in unblocked diaphragms. They were used in lieu of adding shearwalls or braces in the interior.
What the design entailed was that at blocked diaphragms (and unblocked), the sheets would stagger so the edge nailing would penetrate into the field nailed joist below. When the diaphragm was unblocked, we used screws and reduced the capacity based on the penetration into the existing sheathing.
I don't see why you couldn't use the same methodology on shearwalls. In reality, I'm not sure how effective it is, especially in the unblocked instance since you still rely on the original nailing to transfer some shear.
Also, if your shear gets too high, I'm not sure how you handle all the 3x min. thickness issues for studs & plates in the new code. Maybe if you restricted the new fastening to 4" o.c. or greater, you might be able to get by.
Gerard Madden, P.E.
Middlebrook + Louie, Structural Engineers
71 Stevenson Street, Suite 2100
San Francisco, CA 94105