asiatic I still think it would be easier to convey with words and actions than a ring.
I mean, a wedding ring doesn't deter some people though it officially conveys "unavailable" so even if the black ring on the middle finger was widely understood, it wouldn't necessarily serve the purpose you want it to.
Given that it isn't widely understood, it's not going to be useful in the way you want it to.
I think asexuality probably is poorly understood or recognised but I think that's a separate issue , really, to you conveying to someone "I am only interested in you as a friend". I don't think your sexuality needs to be brought into it, necessarily. Your reasons for not fancying someone are often not relevant and, where they are relevant, where you do want to explain that you are asexual, then you should do just that but a ring is not going to convince someone more if they don't believe you and it's not going to work as a signal from the beginning as it's not widely known.
Just jump in there as soon as you can with a statement about how you are defintiely not looking for any kind of relationship other than friendship when you can and then the reasons for that can be revealed, or not, as the friendship goes on.
I mean, most meetings of people are not on the basis taht you are expecting or hoping to get involved with them anyway.
Is your identifying as asexual a relatively new thing? It sounds like it isn't but I just felt that, if it were, it might be why you wanted to get it out there early and also that you are grappling with the implications and therefore over-thinking the issue a bit as I really don't see how such a symbol is going to be generally or regularly useful even if it did work.