I made a modified version of
TeX Gyre Schola (LPPL) in fontforge, and embedded both the regular version and the modified version in an ePub. Editing the font was easy, since you can set the LBearing/RBearing of an entire span of glyphs in the font with a single command in fontforge. So it only took a couple minutes.
Here's the result. Looks pretty good. There's a minor problem with words at the end of the beginning of the line--see the k e i n e at the end of line 2 and U r s a c h e at the beginning of line 3 and how they're not flush with the edges, but I'm fairly sure I'd have the same problem with spans around each character and adding to the margin. I think I can live with that.
The real problem is with ePub renderers that don't support embedded fonts. I gather this includes the damned iPad! I suppose I could define this font as the italic version of a new font family, so if it has to fall back, it'll fall back on italics, but I seem to remember ADE having a problem whereby it would oblique-ify already italic fonts if they're set as font-style: italic;, resulting in an ugly mess--is my memory off on that?
Thanks again to Charleski for the idea...