@KevinH: I can confirm that ePub3-itizer never remembered the target folder and I mention that it irritates me too
The default target folder is not the happiest choice for Windows users.
It can be solved:
1) add a configuration for the plugin where the target path for epub3 files would be saved
2) without configuration: it is enough for the plugin to read the "[mainwindow]" section from the sigil.ini file and take the path to the first element from the "recentfiles" entry.[*]
[*] It is possible to read the path to the currently opened file (as long as user does not open multiple instances of Sigil and edit many files at the same time).
@AlanHK: Your suggestion to add to OPF file
Code:
<meta property="ibooks:specified-fonts">true</meta>
probably fits better with the AddiBooksXML plugin by
@DiapDealer, which can be extended with this option.