%#! lualatex
\documentclass{article}
\usepackage{fontspec,pdftexcmds}

\input luatexja-core.sty

\ExplSyntaxOn
\xetex_or_luatex:nnn { \tl_set:Nn \l_fontspec_pre_feat_sclist }
  {
    \bool_if:NT \l_fontspec_icu_bool {
      \tl_if_empty:NF \l_fontspec_script_tl
      {
        script   = \l_fontspec_script_tl ;
        language = \l_fontspec_lang_tl   ;jfm=ujis;
      }
    }
  }
  {
    mode     = \l_fontspec_mode_tl   ;
    \tl_if_empty:NF \l_fontspec_script_tl
    {
      script   = \l_fontspec_script_tl ;
      language = \l_fontspec_lang_tl   ;jfm=ujis;
    }
  }
\ExplSyntaxOff

\newopentypefeature{Style}{OLDJIS}{+jp90}
\newfontfamily\goth[BoldFont={ipaexg.ttf},Style=OLDJIS]{ipaexm.ttf}

\begin{document}
\makeatletter
\def\extract@font{%
    \get@external@font
    \ifnum\pdf@strcmp{\f@encoding}{EU2}=0
      \expandafter\globaljfont\font@name\external@font\relax
      \font@name\global\zw=\zw\global\zh=\zh
    \else
      \global\expandafter\font\font@name\external@font\relax
      \font@name
    \fi
    \ifnum\pdf@strcmp{\f@encoding}{EU2}=0%
      \ltjglobalsetparameter{kanjiskip={0pt plus 0.4pt minus 0.4pt},
        xkanjiskip={0.25\zw plus 0.083333\zw minus 0.083333\zw}}%
    \fi
    \csname \f@encoding+\f@family\endcsname
    \csname\curr@fontshape\endcsname
       }
% ! The font "ipaexg.ttf/I" cannot be found. というエラーが出る


\goth abcdfghjfgあいう辻）\textbf{\unkern（辻あ{\Large あ}いう}

\goth abcdfghjfgあいう辻）\textbf{（辻あ{\Large あ}いう}
% \unkern がないとうなくいかない

\tracingonline=1\scrollmode\showboxdepth=10000\showboxbreadth=10000\showlists
\end{document}