Dicho:

I've checked the HTML 4 Spec, and the T-overbar is not a defined character for html.

Also checked as much as I could of the Unicode online standards, and it does not appear as a character in any language using Latin style characters, as far as I can tell.

Unicode defines "combining characters" that can be used in a program, to allow you to, essentiall, overtype one character on top of another. As far as I can tell, there is no direct implementation of this in html.

HTML does allow for defining special characters, but it appears that the definition must go in the document header material - or it must at least be enabled there. Someone else may be able to educate us on that.
Since a post to mudcat is an "edit" to a standing document, we don't have access to the header to define "new stuff," and it certainly would involve more than just typing a character code.

Since the character(s) you're asking for don't appear in Unicode, they are not likely to appear in a major language? Perhaps if we knew more about why you need this, someone would be able to offer more help.

John