Sorry to correct you, but PRS+ is not available for models 700 and 900.
But you may search on the-ebook.org
for a "russification package by boroda (aka porkupan her at MR) or amutin. - Use google-translate if needed.
This mod fw is primary meant a enably support for Cyrillic, but it embedds a modified font, maybe supporting also your missing glyphs.
Otherwise I see no other way as to go with CSS and calibre.