I haven't looked at a set of Shimano barcons in years, but if they still use 3 sectors, it may be easier to modify the sectors than the bar.
Remove the expander bolt, and take the sectors out from the spring. Be gentle so you don't destroy the spring. Then file or sand a bit off the mating ends of each sector. This allows them to fit a bit closer together making the minimum OD smaller.
Reassemble and you should be good to go.
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