My suggestion would be if --code-page is given, transliterate to ascii all characters that fall out of the code page range. I.e. if I like build Cyrillic map (--code-page=1251) I would transliterate all characters that fall out of (0x0020-0x007f; 0x0400-0x04ff) ranges, so If any i.e. Greek characters are present it the input data, they will be "mapped" to ascii. Similar to with cp1252 (i.e. latin1), transliterate everyting outside 0x0020-0x007f; 0x00a0-0x00ff ranges. cp1250, transliterate everything >0x007f.

Cyrillic, in most cases, is easy to transliterate (or romanize, etc) by simple mapping as there are few exceptions. Try properly transliterating i.e. Greek (i.e. http://en.wikipedia.org/wiki/Romanization_of_Greek), taking account letter combination and positioning in the word, assuming you know which standard of Greek romanization you will use...

What I did to work around it is to use small program that pre-processes osm data, adding name:ascii tags whenever it encounters non-ascii characters in the osm data, and than use --name-tag-order=name:ascii, name as options. However, it think that similar functionally could be implement in mkgmap...

Regards,
Ivan