That is "AT", isn't it?

(No, it isn't, if you look closely enough.)

Yeah...what I'd really like to do would be to give a character a "natural" background color, e.g.

Then its simple for support to say "red is one, green is ell". But you can't just add a color to a character, because copy paste/rich formatting don't work everywhere, or even transfer well...

Alternatively, if you use ⓪ and ⒈ it matters less if the user says "at" or "zero", and more that they didn't say "oh" or "one".

It's obviously a �. Or perhaps a □ . Maybe an ¾, on odd Wednesdays?

(Unicode has its strengths. Making up replacement characters isn't one.)

I happen to know that the biggest ski resort reservation system in Scandinavia contains a function called MaybeOnATuesday(), but to my knowledge it's never called.

If you are trying for URL safe, Unicode is problematic because of Punycode conversions and differing browser behavior with Unicode URLs. (Some browsers always show Unicode as Unicode in URLs. Some browsers always show Unicode as Punycode in URLs. Some browsers switch between the two based on a huge number of variables such as gTLD, user preference, phase of the moon, etc.)