The 1.8 branch of the Nvi editor, as well as the Nvi2 fork have mulitbyte support.
I could be mistaken but I believe both of these multibyte implementations descend from the late itojun's (http://www.itojun.org/itojun.html) Nvi-m17n project (archived at https://cgit.freebsd.org/ports/tree/editors/nvi-m17n/files). A paper was presented at USENIX99 (http://www.usenix.org/events/usenix99/full_papers/hagino/hag...) describing the work, which was actually developed as part of the KAME project.