Elevator Shaft Shear Wall - NYC Brownstone

This is not just a NYC problem. You are expecting too much from blockwork. Design the walls with reinforced concrete.

Note that depending of you story height, 8 inch might be too thin for wall stability concern !

@brownstonestruct: Based on the brief information you have provided, I would agree with hokie66. But you can make it work if you want to......
A lot would depend on the location of the new elevator core in relation to the combined structure. The core placed symmetrically about the combined structure is desirable. The core located at the perimeter would be problematic. What is the new floor system / framing? The new floor system will have to transfer the lateral load to the core through a combined action of diaphragm + collectors. You mentioned the shaft needs to be designed to resist the total seismic lateral forces? Of course, you can design conservatively, but then what was the LLRS earlier?
As far as reinforcing goes..... I don't know which code you follow. I have ACI 530-05. Chapter 3, section 3.3.3.1 gives the bar size in relation to member thickness, least dimension of the cell, and area of the cell. But codal provisions apart, we always reduce it further by two sizes; max #8 in 10" wall, max #6 in 8" thick wall. We have never used anything more than #8 rebar in masonry construction. We try to restrict the projections of the bars above the slab preferably to less than 3'- 6" from block placement point of view.
Per section 3.3.3.6, bundling of the bars is not permitted. We don't ever attempt welding of rebars or use of couplers.
Why place everything at the ends? In design, we account for all the vertical reinforcing. This way rebars required at the ends can be significantly reduced. You mentioned 8 - #9 at corners. Do you have return flanges along the elevator doors? If not, insist on it, that would help in the placement for 13 feet walls.

Thank you all for your replies, especially DST148. Regarding the old lateral system, it never really had one except for sketchy unreinforced brick. Now, due to the size of renovation, the building has to withstand 2/3 of the full seismic force per NYCBC. Due to several door openings at critical locations it only makes sense to use concrete at the lower floors and switch to CMU above where there are less openings and the forces are lower. I did a FEA model of the shaft and found high stresses in the CMU "headers" or "link beams" above the door openings and am studying those areas more carefully now as well. Also, the shaft is located not at the center of the structure but along one of the party walls not permitting the use of CMU wall along that side effectively giving a large "C" shape shaft that will be pinned to the existing 12" brick party wall.

Because the shaft is not central, it will be subject to torsional loading. A "C" shape is not very good in resisting torsion, and it is probably unwise to try to make the new shaft composite enough with the brick for both to act together. I think you should consider making the complete rectangle of the new shaft in concrete (or block if it will work), so you would have a new wall against the existing brick on that side for the extent of the shaft.

@brownstonestruct: Link beam in masonry is rather unusual. If you want to take advantage of the masonry above elevator door openings, then a masonry divider wall(in lieu of steel / concrete divider beams) between the two elevator shafts would definitely help a lot.