I think there's three options - all have issues as my mate is reluctant to take the head(s) off.
1 - Over size the top portion of the rocker pedestal thus creating an area around the bolt that oil can flow through (per std arrangement).
Issue - Need to somehow prevent getting filings everywhere, incl down the oil gallery.
2 - Machine the Bolt Down on a lathe in the area where the oil flows out of the pedestal.
Issue - Need to ensure bolt retains enough strength to effectively hold rocker shaft.
3 - file back the oil gallery so oil flows straight into the rocker shaft.
Issue - Unclear whether this will provide sufficient oil.