As I've proven in this topic (warning, it's in French), there do in fact exist official Lombax signs for the letters K and X (also in my new sig). Since these come from observation, there could or could not be usage of J and Z in the series.
Which is why I propose this task:
Capture any Lombax writing you spot throughout the games and comic, and attempt to translate them in hope of spotting unusual symbols that could be J or Z.
I think there's a purple sign with Lombax writing in Luminopolis but I haven't been able to get a proper picture of it. Also, I don't have the comic books either.
Reminder: the unofficial signs for J and Z are respectively and