Thought I'd found a neat and relatively cheap solution by buying some tioga R60 risers. High 60mm rise and an almost identical sweep to the M-bars - if anything slightly flatter.
Anyway, I took the M bars off and tried threading the risers through the clamp, got it nearly all the way round the 2nd bend but could get it no further despite greasing, cajoling and cursing. It would have looked mint as well in all back and given a slightly lower bar height.
Back to the drawing board anyway, I may resort to the aberhallo stem thing and a shallow riser or flat bar though it won't be quite as elegant. The tioga bar was the highest rise one I could find although there are plenty that go to 50mm. The problem is if they start bulging before the bend then its going to be tricky to thread them through. Damn you brompton and your esoteric stem design!