It is that MySQL 5 cannot support characters outside the BMP at all, or just that it can't collate them properly? If it just handles > BMP UTF-8 sequences as binary data, might it simply sort them in Unicode code point order?
Or does it do something worse, and actively convert the Unicode characters into a 16-bit range, thus nuking characters outside the BMP. rather than storing, and largely processing, them as binary-encoded data for purposes other than collating?
-- Neil