What's about embedding a/the font(s) which include these special glyphs in the epub?
Or with copying this font(s) to your reader and pointing the calibre CSS styles to this font(s)?
Both is discussed many times, here and in the calibre section of this forum.
HTH
Mark
PS: You may also try a russified
Custom
Firm
Ware (like Boroda/Porkupan or Amutin) or PRS+.
In any case the internal fonts will be replaced with an extended set.
You can check the available glyphs
here