[GeoffR]

I don't know if this will work, and even if it does then there might be side-effects for Japanese or Chinese language books, but if someone wants to try it and find out ...

Freedom to advanced fonts control, paste into libnickel.so.1.0.0.yaml:

Spoiler:

Freedom to advanced fonts control:
  - Enabled: no
  - Description: |
      This patch allows the advanced font options (weight/sharpness adjustments)
      to work with sideloaded fonts too. (KEPUB: OTF and TTF. EPUB: TTF only.)
      Beware unknown side-effects for Japanese or Chinese language books.
    #
    # The following passes a junk pointer to N3FontTypeUtil::setupDropdown, but it
    # seems the pointer is only passed on to ReadingSettings::getDefaultFontFamily
    # where the code is patched to avoid dereferencing it, so it should be safe.
    # BEWARE: What are the side-effects for Japanese / Chinese language books?
    #
    # N3FontTypeUtil.setupMonotypeDropdown(b,c)
    # --> N3FontTypeUtil.setupDropdown(UNDEFINED,b,c)
  - ReplaceBytes: {Offset: 0x8432E4, FindH: 59 46 50 46, ReplaceH: 5A 46 51 46}
  - ReplaceBLX: {Offset: 0x8432E8, Find: 0x4073B8, Replace: 0x41F440}
    # N3FontTypeController.setupAdvancedOption(...) --> NOP
  - ReplaceBytesNOP: {Offset: 0x9FB23A, FindBLX: 0x422810}
  - ReplaceBytesNOP: {Offset: 0x9FC1D0, FindBLX: 0x422810}
    # Ignore volume language (affects language number 25; ja? zh?)
  - ReplaceBytesNOP: {Offset: 0x65F7EA, FindBLX: 0x415B74}
  - ReplaceBytesNOP: {Offset: 0x65F7F0, FindBLX: 0x41D354}
  - ReplaceBytes: {Offset: 0x65F7F4, FindH: 06 46, ReplaceH: 00 26}
  - ReplaceBytesNOP: {Offset: 0x65F7F8, FindBLX: 0x40E5B4}