the ePub generated from http://www.perseus.tufts.edu/hopper/opensource/downloads/texts/tei/1999.04.0074.xml using OxGarage had another issue
The resulting ePub document doesn't contain any embedded font. I'm afraid that ereaders (with eink, not tablets) don't allow installing fonts or anything (unless you root or jailbreak them [which might be illegal; not 100% sure about this]).
The vast majority of ereaders may display the Greek and Russian Unicode blocks (and the ones for Basic Latin and Latin-1 Supplement), but not the Greek Extended block. Actually they don't display the Latin Extended-A block (so ō is displayed as a question mark when the font isn't embedded). FreeSerif (or Sans) may be a solution to this (free licensed font [GNU GPL] and available at http://ftp.gnu.org/gnu/freefont/\).
My suggestion to this issue is: would it be possible that the transformation itself detects the font has to be embedded and embeds it
in the ePub file? (I mean, when there is a character from the Greek extended Unicode block in the text.) Or at least, it would be useful that there is an option to embed the fonts.
Many thanks for your help,