EDIT: Doesn't work completely, see comment on Muihlinn's answer.
Based off Muihlinn's answer, I looked into prettify-symbols-mode and ended up finding this macro which solved my problem cleanly: https://github.com/Ilazki/prettify-utils.el/blob/master/prettify-utils.el#L222-L243
You could use prettify-symbols-mode with something, evaluable in your sratch buffer, like this:
(push '("2/3" . ?⅔) prettify-symbols-alist)
It only adds that particular fraction and it's unicode counterpart to the list of symbols to prettify, then resets the mode
If it works as expected, ...
If you're willing to have the text change, you can use one of Emacs' many input methods. With e.g. rfc3145, you could type that as
or with TeX it would be
Use C-\ to choose your input method.
(set-display-table-slot·buffer-display-table 'wrap ?↩) works for me, with one caveat. In a terminal the arrow shows up correctly, but in the gui (with fringe-mode turned off), the glyph is drawn with a replacement character; apparently it couldn't find a glyph for the arrow in the chosen font. It does render correctly in the scratch buffer, but it's using a ...