IDW doesn't have a BOM.
It can have a part list, but that Part List is the result of the 3D BOM.
The Item Number is an assembly property, not a part property. Example, you can have the same part in different assemblies, with different Item Numbers.
So you can't pull out that property in a part, since the property is not there to pull.
So, you have to create it (custom iProperty)... But... and going again to the previous example, this workflow has flaws: you cannot use the same part in different assemblies, because the custom property will change in the part depends on where you run the rule: on Assembly A or assembly B. Also the library parts will not work, since you can't modified and save them (to create the custom parameter).
I Understand that is handy to get the Item number to use as an identifier in drawings, but is not easy to get it, and in most of cases is workflow dependent, that doesn't work for every situations, only for that specific case.
In my case, the workflow i suggest is used in specific situations: where the parts involved are only used in that assembly model and i don't have any library components...
CCarreiras