I can't say for sure but at the gun counter where I work there's a SIG .380 (a P238) with a hand engraved slide. They just cut through the normal black coating and then gold-plated (I'm pretty sure it's just plate vs inlay) a flower. Looks really nice with the bright silver cuts against the black but I always explain to people that you only do that if it's your gun or you're at the factory because one little slip and...
In any case I'm guessing the guns are engravable but if you can get access to them and do a test cut somewhere it won't show you'll actually know.
Most factory engraved guns are engraved before the finish is done. Some of those finishes make the gun really hard. Like Glocks. If they have the tenifer (spelling) Nothing will cut it. Best way to find out is to call the factory and ask them. Hope this helps.