So I tried one of the T S Eliot samples with embedded fonts from
IDPF's github.
The embedded font wasn't used in either version of PB Reader.
I figured it out!
The HTML didn't specify any font at all apart from the custom paragraph. The reader app was set to use my current favourite font, and that seems to be overriding
everything in the book. That seems a bit heavy-handed to me.
Changing the reader to use the "default" font (whatever that might be, some kind of serif font) and ... the embedded font renders.
Using "!important" in the CSS using the embedded font does not help.
This has been a nice little detour. Plenty of potholes and roadworks, to be sure.