The previous post mentioned the problem of the absence of a variant of the Gentium font which does both bold and italic. JT suggested Garamond as an alternative, and I found the Garamond free download (GaramondNo8 from http://garamond.org/) does have this variant, and seems to make a passable substitute. Using it involves putting the Garamond font in the XEP fonts directory, and changing this:

    <font-family name="GentiumReg" ligatures="fi fl" embed="true" subset="true">
      <font><font-data ttf="GenR102.ttf"/></font>
      <font style="oblique"><font-data ttf="GenI102.ttf"/></font>
      <font style="italic"><font-data ttf="GenI102.ttf"/></font>
      <font weight="bold"><font-data ttf="GARABD.TTF"/></font>
    </font-family>

to this:

    <font-family name="GentiumReg" ligatures="fi fl" embed="true" subset="true">
      <font><font-data ttf="GenR102.ttf"/></font>
      <font style="oblique"><font-data ttf="GenI102.ttf"/></font>
      <font style="italic"><font-data ttf="GenI102.ttf"/></font>
      <font weight="bold"><font-data ttf="GARABD.TTF"/></font>
      <font style="italic" weight="bold"><font-data ttf="GaramondNo8-Bold-Italic.ttf"/></font>
    </font-family>

in xep.xml.

Notice that we're already using a Garamond font for bold, which I'd forgotten. I've tested this locally, and it works; I also made the same change on the server, but it hasn't taken effect yet; presumably it will require a restart of Tomcat to cause all the configs to be re-read, and it's not worth restarting just for this.