Putting your name on synths is one of the many thing I can't for the life of me even fathom why the hell they didn't just automatically ported. Like the sorting. And the Targeting. And the Map Markers. And the calls. And the-- I'm gonna cut it right here, this could get long.
Now the adding a personal comment thing? That could be rather entertaining, I love it. I don't know if it was Tanaka's department, but now that the whole team if different, I can actually hope for this to make it. I know the last team would've given us something really cheesy, like the Chocobo, Wyvern and Automaton naming in FFXI, picking the name from a list. What a horrible thing, if you want to stop "bad" things from being "said" in those items or pets, then just use the same kind of control you use for character names or chat. Hell, it's not like you won't be able to track the user, it has it's name on it...
If "space in the item window" is supposed to be a problem, heck, add a "Check" or "Inscription" option when selecting an item that pops a new, independent window dedicated to just reading what our crafter put in there and a new icon (like the EX and whatnot) to know when an item has an inscription/comment/signature. Because it'll make us feel good. It'll make me feel good.
Edit:
I don't quite understand why putting your signature on your work has to be rare at all. If I want to put my name and/or add "Scabbard sold separately" on every sword I make I should be entitled to. I mean, I just made it! Come on!
If anything, what came to my mind is that they had mentioned before that they don't have "many ways to take gil away from the game". This signature option could be enabled via a buff from the NPC's that provide facilities. It's a silly/fun thing... But I think I'd still throw gil at it from time to time, as long as it doesn't go thinking I am paying for a Dynamis entry, keep it civil, crafters love their coin.