\documentclass{ltjarticle}
\makeatletter
\def\mdseries@mc{el}
\def\bfseries@mc{sb}
\def\mdseries@gt{l}
\def\bfseries@gt{k}
\DeclareFontShape{JY3}{gt}{k}{n}{<-> KozGoPr6N-Heavy.otf:jfm=ujis}{}
\DeclareFontShape{JT3}{gt}{k}{n}{<-> KozGoPr6N-Heavy.otf:jfm=ujisv}{}
\DeclareFontShape{JY3}{gt}{l}{n}{<-> KozGoPr6N-ExtraLight.otf:jfm=ujis}{}
\DeclareFontShape{JT3}{gt}{l}{n}{<-> KozGoPr6N-ExtraLight.otf:jfm=ujisv}{}
\DeclareFontShape{JY3}{mc}{el}{n}{<-> KozMinPr6N-ExtraLight.otf:jfm=ujis}{}
\DeclareFontShape{JT3}{mc}{el}{n}{<-> KozMinPr6N-ExtraLight.otf:jfm=ujisv}{}
\DeclareFontShape{JY3}{mc}{sb}{n}{<-> KozMinPr6N-Bold.otf:jfm=ujis}{}
\DeclareFontShape{JT3}{mc}{sb}{n}{<-> KozMinPr6N-Bold.otf:jfm=ujisv}{}
\def\TEST{%
 \par
  A: \f@encoding/\f@family/\f@series/\f@shape.
  K: \k@encoding/\k@family/\k@series/\k@shape.
  あいうABC
}
\begin{document}
\TEST NORMAL
\emph{\TEST} EMPH
\textsf{\TEST} SF
\textbf{\TEST} BF
\textgt{\TEST} GT
\textbf{\textgt{\TEST}} BF+GT
\textbf{\textgt{\textmd{\TEST}}} BF+GT+MD

\textsc{\textit{\TEST}} SC+IT

{\fontseries{b}\selectfont \TEST} B
{\fontseries{bx}\selectfont \TEST} BX

\end{document}