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The conventional semantics of "ignore" would be "remove the
failing characters from the output". This would be difficult
to implement if the Microsoft API provides no detailed error
indication.

You could try to get more detailed error indication by
re-encoding the resulting string with a NULL buffer,
counting the number of characters that have successfully
been encoded, atleast in the .decode case. 

In the .encode case, you could try using \0 as the default
char. To my knowledge, no ACP ever uses \0 in a multi-byte
string.

What is the meaning of the WC_DEFAULTCHAR flag, in
WideCharToMultiByte, and why are you not using it?

I'm somewhat concerned with backwards compatibility, since
the mbcs codec has never returned errors. So this should be
applied to 2.4 only, and listed in whatsnew.tex.