Can cast iron with non-metal components be seasoned?
Le Creuset say 450°F/230°C is the maximum temperature their enamelled cast-ironware will take. Other manufacturers seem to be sometimes lower, though that could be a handle/lid requirement rather than the enamel itself - see Why are most enameled cast iron dutch oven&#39;s only rated up to 400-450 F?

Your problem, assuming you season the mortar with a low-smoke-point oil is that your pestle is going to have to be carefully done over a flame. That handle is not going to survive in the oven. If it's silicone, then about 180°C is about all it can take… that's if you're certain it's silicone.

 I'm gonna have to edit that apostrophe out of the linked question, it's burning a hole in my OCD ;)