So I had the intention of having a custom martin 000 built as a tribute to someone very close to me who passed away last year. I wanted the guitar to be all in black, with a completely bare fretboard except for that persons signature. However, I ended up finding a 000 that I ADORED the sound of, and bought it instead. My question is, would it be possible to take this guitar to a luthier and have him inlays this signature on the already-finished guitar? I know a completely new fretboard could be put on it, but I'd like to keep the guitar as original as possible.
There's an Ebay seller who makes custom 12th fret stickers to your order, jocomo81 there has many other decal stickers for use on a guitar. This guy could make something for you. His stickers are thin and removable, so the value of the guitar isn't hurt if you use them.
Real inlay on an existing guitar will be expensive and it might be hard to find someone who will do it.
Good luck
Might be pretty difficult on a finished guitar. There's not a lot of space between the 11th and 12th frets.

What they might do is remove the frets, do the inlay, sand it to match the radius of the board, and re-fret it. It won't be cheap, though.