After some googling I think I've determined that PocketBook doesn't support fonts embedded in EPUBs at all, which I suspect then carries over to not supporting them in SVGs. That's really quite annoying. Surely the RMSDK supports this?
One question might be, if I installed a font in the usual system/fonts directory, can I refer to it in CSS, either with or without a @font-face? This would make the books completely non-portable, but would be a potential workaround.
Separately, I noticed that <pre> does not render with a monospaced font. But <tt> does, which is a bit surprising.
So "<pre><tt>...</tt></pre>" it is...