Bivalent file streams with element-type eq to simple-char?

Hello lispworkers,

Though bivalent streams were announces in LW 4.3, I have failed to find any
mention of them in documentation. Experiments revealed that they do work for
files opened with element-type equal to base-char, which are of use with the
external format latin-1. But they seem not to work when element-type is
simple-char needed for win32:*multibyte-code-page-ef*.

Is this observation correct? Any workaround or hint?
--
Sincerely,
Dmitri Ivanov
lisp.ystok.ru