Okay, I've figured it out, here's how to do it:
The game should have a LICDIR/LIC.DAT. If it doesn't, you might be able to generate it, use google.
First you need to "prepare" the game like you would for backup injection using something like
CFW2OFW Helper.
Simply put the PS3_GAME folder in the same place as the .exe and run it. It will download and unpack the updates by itself.
This should give you 2 folders, one starting with BL/BC (BLUS, etc) and one starting with NP (NPUB, etc).
Then you need to download
this tool, place the BL/BC+NP folders in the same folder as the .bat, and run the .bat.
This should make 3 pkgs, all starting with 000000-NP.
Run these 3 pkgs through ps3xploit_rifgen_edatresign.exe to obtain _signed.pkg versions.
Install all 3 of these on your PS3
without Debug PKG enabled.
You should now be able to play!