Is the implication that there is some extraordinary property to Kaido's club?
Don't see why not. Especially if it was made on Wano. With the fire splitting swords and everything. Hope it's something more interesting than seastone.
The seastone info explains why Dofla had so much of it, and he was most likely the primary supplier to WG.
That silhouette definitely doesn't look like any of the samurai shown. Looks like Vegapunk's silhouette. Since it seems unlikely it's Vegapunk himself, maybe it's his brother, or something. Then again, if Vegapunk got himself captured by Kaido, it's not like WG would announce it to the whole world. And Kaido did just lose his source of SMILE.
Vegapunk being there might shed some light on the whole devil fruit thing, finally. There's not too much time left for that. And he could also help his cellmates discover how to awaken their devil fruits. Convenient, huh? Not to mention it would give marines a reason to be on Wano. If you think about it, it could all fit nicely. In chapter 905: A Beautiful World, Ryokugyuu mentions Fujitora was on a long trip, and asked how Vegapunk was doing. That kinda implies Vegapunk is not in Mariejois or any of the two close bases.
Last night I suddenly rememberd Sentoumaru, and how he might appear in Wano, considering his name and likeness. And now there's even one more possible connection, seeing how he was working with Vegapunk. And he even appears on the damn cover.
So I do hope it's Vegapunk himself, yes.