Incremental THAC0 bonus affecting ability to hit creatures immune to normal weapons

I am trying to figure out a way to implement the original Kensai Kit from Oriental Adventures 1st edition

The challenge remains trying to implement the Kensai's incremental THAC0 bonuses specifically the part that allows this bonus "to hit creatures that normally can be hit only by magical weapons" i.e. Monk Fists

I attempted to modify the enchantment and magical flags of various items, but testing my theory using a flesh golem was unsuccessful as they can only be hit by an item flagged as "magical" regardless of the enchantment value.

Could I use the apply effect on equip item (182) effect? and create an EFF file?

or THAC0 vs type bonus (178) and use SPECIFC.IDS (Magic - 101)?

Override creature data (342)?
Any ideas?