It looks to me like pg_wchar2utf_with_len will not work, because
unicode_to_utf8 returns its second argument unmodified - not, as your code seems to assume, the byte following what was already written.
Fixed.
MULE also looks problematic. The code that you've written isn't symmetric with the opposite conversion, unlike what you did in all other cases, and I don't understand why. I'm also somewhat baffled by the reverse conversion: it treats a multi-byte sequence beginning with a byte for which IS_LCPRV1(x) returns true as invalid if there are less than 3 bytes available, but it only reads two; similarly, for IS_LCPRV2(x), it demands 4 bytes but converts only 3.
Should we save existing pg_wchar representation for MULE encoding? Probably, we can modify it like in 0.1 version of patch in order to make it more transparent.
