It appears that the new suggested edits feature was only partially implemented with tag wikis. I experienced this problem on SuperUser, but suspect it's a system-wide issue with the engine, so am bringing it up on MSO.
I was given the option to edit the tag wikis for tags which already had one, but upon submitting a proposed edit, was greeted with an error screen every time.
Tags which do not yet have a tag wiki have no means by which to propose a new one entirely.
I'm not sure to what extent this is expected behavior and to what extent this is a bug, but it's certainly not behaving consistently.