has anyone done the equivalent of a Costed Bill of Inquiry for Average cost assemblies when Advanced Bom feature is turned on?

No because average cost changes with every receipt, build, etc. The Cost BOM Inquiry is based off a Planned standard Cost record which is static

Cheers, yes have created a saved search. If not using routings and labour being recorded via non inventory items, then non inventory items don't have an average cost, only purchase price + UOM conversion for minutes.. gets messy quick!