It turns out, yes! You can use symbols like π in neocities sites.
URLs work on something called Punycode. Basically, if the URL isn't pure ASCII, it becomes Punycode. I don't really know how it works, but a single π turns into xn--1xa in Punycode. So, I made a Neocities account called xn--1xa, and sure enough, it worked!
My main website can be found here.
