We are actually actively working on Cyrillic-ASCII transliteration for glibc in this very moment. Please check this patch for details [1]
Your help and suggestions welcome to make sure this will be a useful and consistent fix when it lands.
The bug [2] is from 2006 (sic!) and it is a reason why transliteration may not work with iconv for some systems/locales.
[1] https://sourceware.org/ml/libc-locales/2019-q1/msg00010.html
[2]https://sourceware.org/bugzilla/show_bug.cgi?id=2872