I'm afraid that is not up to us
Conclusion: I have to create at least 30*(other-combination) unique macro blocks.
I think it is not so good for performance. More important, it looks dirty.
If we were to make it the way you propose (per Item), performance would still be bad, because under the hood APIs and GPUs don't like having the depth bias changing that often(*).
By making it part of Macroblock, we make it more evident that it is slow.
(*)Some GPUs have no problem, some do (but still the API works that way, so the CPU cycles will be consumed)
Also please beware depth bias is not well defined. While most GPU vendors seem now to agree how depth bias works now; there is not a clear definition of what unit of measure depth bias is; thus making rendering inconsistent between GPUs. It was exposed because "a little bias" tends to workaround Z-fighting. Depth Slope Bias is clearly defined though and thus doesn't have this problem.