In Russian, the difference between Е and Ё is primary at the beginning of a word as they are considered distinct letters of the alphabet, yet secondary in the middle of a word, as the dieresis over Ё is not mandatory. As an example, ель < ёлка, but тёлка < тель, see http://ru.wikisource.org/wiki/Орфографический_словарь_русского_языка
A cursory scan of the UCA doesn't reveal if that's implementable, and experiments in a fairly fresh Linux Mint yield either ель < ёлка < тель < тёлка or ель < тель < тёлка < ёлка depending on the LANG setting (en_US works better than ru_RU). Could someone tell if the UCA in its current form is able to support that? Thanks, Leo