Rather than simply quietly inserting pawns and other placeholder characters into the page text, VisualEditor should do a sanity-check pre-save and either outright reject or loudly warn about edits that will insert such characters.
Version: unspecified
Severity: enhancement
I suggest piggybacking on the existing sanity check code: display the "your edit may be corrupted" warning in the save dialog and tag the edit with visualeditor-check.
The problem of inserting pawns and such is ancient history now, so building a system to detect them is not a good use of time.