another multibyte question

Do any of the encodings with encoding max length > 1 have a constant
character size (e.g. unicode?). If so, how hard would it be to add
another member to pg_wchar_tbl, say:

bool mblen_is_const; /* all chars = max bytes this charset */

Then those character sets code gain back much of the same speed
advantages as single byte character sets when it comes to string processing.

Joe