I think epub shall be made with one font - otherwise it will be too big if fonts are embedded. So your interface with one font embedding seams quite reasonableas for me. Just make this checkbox active and embedd the font selected in the box.
To protect fonts copyrights, you can allow to export only free fonts, such as DejaVu and other fonts developed by FontForge - they are included anyway into OO package.
About TOC

It's my mistake.

W2E did indeed automatically produces an hypertext table of contents which works very well. I understand it's the easiest solution. Forgive my mistake.

I confirm that it's useless for a OpenOffice writer user to create a TOC. It would only duplicate the epub one.

I send you the book. The image is in page 2 and is totally unrelated to the book. It's just a trial.
There is a problem… The fonts listed in the preferences* are the fonts available in the system. Not all fonts installed in the system are .otf fonts.
Uhmm… I can list only .otf fonts, when I check the box… why not?

Luke

*in the preference window the font list can be discontined. It's a known bug, and is also the cause of the long delay in the window's opening.
I correct this bug when I start to work to fonts embedding.
eBookLuke,
But why only otf fonts? The most of them are not free.
The best way would be just to include in the extension a disclaimer stating that if somebody will want to sell epubs made with it, then no commercial fonts shall be embedded.
OTF fonts are the only officially supported by ePub. There are a lot of free format converter online that allows to convert all font formati in .otf.
About the disclaimer, it will be not a problem.

Luke
Well, otf fonts maybe officially supported, but I believe a lot of people are using ttf for docs creation - so why to police them to use fonts which they like?
And by not applying otf/ttf filter you can make embedding faster.