@statu:
Huh, I guess I just never thought of it that way, I like that as an explanation as to why they can carve poneglyphs, tho I just always assumed it was a matter of skill / technique purely.
I might be completely wrong here and even if I'm not, this might just be a moot point, but I think Yams spent almost all his time, if not all, on Onigashima cuz of those cuffs? If I understood what you meant correctly? Anyways I think this really just me nitpicking and not really worth discussing much.
No this isn't you nitpicking, the theory is absolutely paper-thin and based on unresolved plot points:
What did the Government want from Orochi?
Was it Seastone?
Does it have anything to do with Wanoese craftsmenship, since Law commented on that delicate seastone nail?
Is the skill to forge seastone connected to carving other super hard materials, like Poneglyphs?
How exactly was Yamato's life in Wano? He was groomed to be incredibly powerful, beaten up numerous times and supplied with a yet to be revealed Devil Fruit. Throwing him into the seastone quarries at one point wouldn't be surprising. I'm not sure, but I do believe Yamato could move within Wano, not just Onigashima (he attended the Oden execution in the capital after all). Kaido limiting his freedom further during adulthood is also possible.
So yeah, there is no solid foundation for Yamato being able to carve stones, but I don't think it's a completely baseless assumption. If Oda wants an Oden-like figure around, that is.
As for leaving message, it might be a cultural thing but it's not really a foreign (or strange?) idea to me. I like it and do think it could be a great moment in the series but I guess it just never occurred to me as necessary. Also this is prob a stupid thing to say after you offered leaving message on poneglyph as an option but if they leave something behind it wouldn't need to be a message on the poneglyph.
It's absolutely not necessary for the crew, they won't be asking Yamato to be their Chronicler or the dude who may or not may carve some maybe existing stone at the maybe existing Laugh Tale.
But I believe we have to look at it from the author's perspective, an author who has to write the final station of a 30+ year long journey. It can't be just good, it has to be as perfect and as memorable as possible. Everyone present needs to be relevant in some way. And if Yamato were to join the crew, then the message-carver job would be his personal climax in all of this, his dream fulfilled.