Turns out several information sources, including
*VERY* detailed guide on changing default Reader fonts and
PRS Customizer itself are partially incorrect: It is a widely accepted fact that fonts to be integrated into Sony Readers need to have their "font family" tag modified, but it turns out
that it is not enough - proper "font family" will make the Reader recognize a font and use it in menu, but it will not use it inside a book unless
a) that font is a serif font (0011), or
b) "full font name" is also modified. Serif font can be left alone because it works even with uncorrected full font name, but sans-serif font requires "Swis721 BT Roman" full-font-name, and monospace font requires "Courier10 BT Roman".
Curiously enough, PRS Customizer's unicode fonts have a proper full-font-name for sans-serif font but invalid for monospace font.