I am having the front of the magwell on mine done right now. The only reason I chose that location was because I wanted to test fire the lower first and then I didn’t want to risk messing it up knocking out the roll pins for the bolt catch and trigger guard. Otherwise I would have done it above the trigger pins.
I would NEVER engrave the barrel on an AR15 SBR. Your lower should last a lifetime; your barrel could wear out or get damaged possibly. It might not even meet BATF requirements depending on how you read the regs.
It does meet the regulations of receiver or barrel. It’s only a hassle when you want to swap uppers. At that point in time, each barrel of each barreled upper will have to have the engraving- even 16" plus ones.