Well, "worth" is in the eye of the beholder... or of the market. I think to gain a premium from the signatures, you would need to find a buyer who holds those people in high enough esteem, and is desperate enough to obtain their autographs on a kendama, to pay a "nice price". My opinion is that the signatures are not rare enough (within the kendama scene) for them to be of premium value.
To be clear, I don't mean to denigrate anyone, just that in my experience, pretty much anyone is happy to sign a kendama if asked, so there's no huge rarity value.
Of course, if you can find a buyer who is willing to pay a nice chunk for it, then go for it. But if those guys signed the kendama for you personally, then perhaps it's worth more to you as a memento of meeting them...?
