Anthraxus
Explorer
I mean, I don't think there is any rule that says you can re-fluff things to the examples I have seen(your Lantan/Smokepowder idea; someone's WH40K space marine). All I've seen is that you can rename a Glaive into "Bill-Guisarme" or use Asian names for your monk's weapons. I have only seen the Admins say "If your DM is ok with it, and mechanical properties remain the same."
In your case, if you had some kind of mechanical smokepowder pistol that Gnomes from Lantan made, and you roleplayed it, and kept it fairly low key, I'd probably be ok with it. If someone in plate mail with a crossbow said he had the "Real Far Traveler" background, and said he was a Space Marine with Power Armor, and his crossbow was a Heavy Bolter, I'd most likely say "Ehhhh, no."
In your case, if you had some kind of mechanical smokepowder pistol that Gnomes from Lantan made, and you roleplayed it, and kept it fairly low key, I'd probably be ok with it. If someone in plate mail with a crossbow said he had the "Real Far Traveler" background, and said he was a Space Marine with Power Armor, and his crossbow was a Heavy Bolter, I'd most likely say "Ehhhh, no."