All those Mass Upload write-back templates you see at several standard workcenters can actually be found at Application and User Management --> Business Flexibility --> Microsoft Office® Template Maintenance.
Similar to forms, these excels are also segregated and maintained at various levels. For eg, Template Group, Language Variant and Template Type.
So, all these std. forms are triggered at their respective wc based on Template Groups and other factors as explained above. Now, unlike custom forms, you can't create a new Template group for excel write-back workbooks to invoke them at runtime dynamically.
Only doc. outputs which get created for custom objects that I know are...
- Form templates.
- OWL Excel sheet