Depends on whether you want to display a custom character (assembly) or token (TI-BASIC). Either way, you'd use a hook (from an assembly app or possibly a program).
For a custom character, you'd use a font hook (the kind zStart and Omnicalc provide). You'd have to replace one of the 255 characters on
this page.
For a custom token, you'd use a token hook (the kind Axe uses). You'd be creating a new "word" out of a combination of the existing characters. You can only replace a token that exists; all 659 tokens can be found starting on
this page.
You can read more about hooks at [wikiti]83Plus:OS:Hooks[/wikiti].