Yes- My Outback grill is.

You don't appear to understand the word "normally" or the word
"porcelain". The overwhelming majority of cast iron grills for barbecue
use are not vitrified enamel, porcelain isn't used at all


Did you try googling for porcelain coated grill?

This site list several. Maybe they don't understand the word "porcelain".

http://www.weber-barbecue-uk.co.uk/a...ue-Spares.html


It is supposed to be heatproof but I have managed
to overheat the grills and cause the porcelain to melt.

That would be impressive, were you using MAPP gas instead of propane?

I use charcoal.