It's just an entry in special_items.csv (enter the weapon, fighter wing, and ship hull tags you want the package to pull from under 'plugin params') and a line in settings.json:
"graphics":{
"blueprint_packages":{
"your_blueprint_package":"graphics/icons/cargo/yourBPImage.png",
},
calling whatever you want to use as a graphic on the blueprint screen. Super simple; you can make a tiny mod, copy the required files from the vanilla game, and change an existing blueprint package's entries.
I've actually thought about making a template mod for this, since it's really not all that difficult (at least in basic form). Maybe I should bang that out this weekend.