Hmmm, that is a super valid point. It really is just a seal that I'm looking for to add on to an existing tanto I have. However, IC I would probably have Mizui write up the blueprint to bring to a craftsman who has Fuuinjutsu to get this piece done being that Mizui doesn't have the Fuuinjutsu FS.
I suppose the bigger question to ask would be if characters can create a custom seal with the intent of finding a Fuuinjutsu user to apply said seal. My assumption, with the explicit wording of characters being unable to create customs for FS's they don't have, is that creating a custom would conflict with said wording. The way I tend to view the item template is a blueprint where you write an intended design and then hand it off to a craftsman and give your trust to them in hopes that the product is as desired.
I definitely want to make sure I'm using the proper channels to get the desired effect for sure.
Thanks for givin' this a look at!
What you'd want to do is explain what you want to said Fuuinjutsu person and have them register whatever seal it would be. This would certainly be an instance of circumventing the rule otherwise - realizing that you did not intend to, of course.