I don't have a Dune to test, but will try to replicate your situation on TViXiE
OK, I created a folder and scanned it with TViXiE(1.3.2b1 configured for DUNE). The contents of the folder are:
there is no folder.jpg but the folder image is actually front_cover.tvixie.
I have successfully managed to change .tvix.aai to incorporate the new folder image using the method I described above.
I also see that when scraping a movie folder with tvixie set for DUNE, no icon appears in folder view. Since I have no Dune, I am unsure if this is normal procedure or not, but at least my method appears to be doing what I expected it to.
The next question is:
Have I understood your question correctly?