TeXmacs does not support unicode natively.

Since TeXmacs font system was built around metafont, the native encoding is Cork, and extended characters are represented as so-called "universal symbols" such as "<pound>" (which is not XML or HTML contrary to appearances).

A large part of the work involved in producing the HTML import/export filters involved writing translation tables between the Cork+symbols system and Unicode.

The first thing you'll need to for native support of something like ccircumflex is to ensure there is symbol defined. Then you need hack the various mysterious tables of font definitions to allow displaying it text mode. Finally hack the keyboard tables to allow input.