In my article Extending the Journal Entry Voucher Upload (Excel), I wrote about Custom Codes (Fine Tuning Activity Account Assignment Types).
Account Assignments do allow additional reporting capabilities, but they are coming with the limitation that they are not available in periodic runs (GR/IR run, WIP clearing, revenue recognition), but there is a way to get this functionality into Fixed Assets.
When depreciation is calculated, automated or manual entries are made for Fixed Assets, it will create a Journal Entry Voucher. The Fixed Asset itself does not support the Custom Code fields, but the Journal Entry does. So, assuming there are enough criterias to derive the Custom Code on the Fixed Asset, you can implement the AfterModify Action of Business Object AccountingEntry.
Snippet:
import ABSL;
this.CustomCode1 = "DE";
