I dont know how it works, but the pkg im talking about I get when I download the license file for the s1 one. The license itself is in the keys.bin file, but I don't know if the keys.bin file has some unique part generated depending on your psp. Anyways, I'll post some later.
Edit: Apparently its not the keys file, sorry for that. Looks like it unpacks two other license files though, really small as well.