I have done an actual unload of one logical document from a load.
But it is a bit of manual process.
1. Extract the document and indexer file using "arsdoc get"
2. Find of which logical document (metadata and offset details from the indexer file you want to unload)
3. Remove the content from the Reportfile(using the offset details)
4. Remove the metadata from the indexer file.
5. Change the offset for the other logical documents.
6. Load this report file and index file again to OD.
7. Unload the Previous load.
I know that this is not someone would suggest. But this helps