Basically you want to add the item # represented in drawing to models. But let me ask you; are your models unique for each assembly? If yes then add the item number already in them and then you can arrange the BOM to show them in sequence. If not then this could create issue at later stage.
Each part should be more or less unique to each ASSM, except for hardware. The problem is when using large ASSM the process you explained is rather tedious and allows for possible user errors.
I'm sure you can achieve the results via macro where once BOM is populated, the macro would create the required property into the models.
I think you'll have to create an Excel BOM to do this.
The "regular" BOM is limited in dealing with the ITEM NO column.
See the formulas in the attachments for this post: How can I change item number format and syntex on BOM
I have to reassert Mr. Gupta's warning on linking to BOM item numbers.
Unless every BOM is locked it's reordered and renumbered.
What if the part is inserted into another assembly and it's at another item number.
I read the thread you posted, thank you. The method described still will not work for me. I can save the BOM and open it in Excel and manipulate it; however, I need the the BOM number to show up in each parts custom properties. I understand this may not be possible and I am needing such a specific output, this is due to an ADDIN for SolidWorks.