Transposed FIR filter structure to compensate for extra delays on addition
Ah, its easier than I thought, just mirror the introduced delays. So this means introduce two additional $z^{-2}$ in the last two stages of the transpose FIR filter structure. After some basic math to get $H(z)$ the coefficients are as below in bd2, which gives the same magnitude frequency response as the original.

bd2 = [0 0 0 0 b(1) b(2) b(3) b(4)];