Manichean's solution works to create the custom column when you use the 'Column built from other columns' and paste it into the 'Template' field and then restart.
I named my custom column 'lastname' for the sort field and Author Lastname to display but you can call it what you wish. Just adjust the 'save to disk' statement I have here accordingly.
Then just put this into the 'save to disk' field for naming the files.
Then I got this file saved to disk for example:
Dickson_Call Him Lord.epub
It still saves the files with the .jpg cover and the .opf file you will have to sort out and trash if you don't need them.
Interesting problem. I learned something today.
The day is a success.