@NiLuJe: The KFX equivalent file is named "CONTENT_FONT_RAMP". But unlike "FONT_RAMP" parsing of that files is disabled on "stock" firmware.
People with jailbreak-ed Kindles can make
one simple edit of appropriate JAR file to enabled the parsing / set "debug" flag to true. (the variable is still named "debug" on firmware 5.10.x, even though there was a new round of code obfuscation).
The best course of action would probably be for all of us to organize and "flood"
kindle-feedback@amazon.com inbox with e-mails asking Lab123 team to just enable that parsing functionality in all future firmwares.