One possible reason is that this would be a deviation from the Birdy design. If Pacific is making Birdys on a license from R&M, they may not approve. I don't think the two bolt holes are a design Brompton owns, at least Tern uses it. I have seen that on a Tern bought in Germany. (This is guessing, I don't know the legal arrangements.) Then again, there is is the titanium Birdy, which is a pretty major deviation.
Another reason is that Birdys can mount two front panniers, so there is less need. But I agree the Brompton style block is a good design. Doesn't affect the steering much.