A possible way would be to use the Workflow External Scripts-App to call an external script to delete the file, if you e.g. rename it to *.delete etc. This would prevent the system from moving it to the trash folder.
Using Flow seems an interesting Idea. I tried around, but you are right. It seems ther is a need for an external script. Do you know a good tutorial for that? Do I have to create a simple bash o PHP script?
Basic information about the app can be found in the Github repository of the app. Due to the fact that several questions around the app usage have already been answered in this forum the past, it would be a good start to check these postings first: