\documentclass[spinewidth=15mm,markcolor=black]{bookcover}

% Loading the glossaries-extra packages won't work here, so LaTeX
% won't understand the .aux file section headers which have \gls
% commands. We resolve this with a little white lie.
\newcommand\glsentrylongpl{}
\newcommand\glsentrytext{}
\newcommand\Glsfmtplural{}
\newcommand\Glsfmtlongpl{}
\newcommand\Gls{}
\newcommand\gls{}
\newcommand\glspl{}
\newcommand\Glspl{}
\newcommand\spellName{}

\RequirePackage{xr-hyper}
\RequirePackage{hyperref}

\makeatletter
\@addtofilelist{../core/main.tex}
\@addtofilelist{../core/core.aux}%
\makeatother

\externaldocument{../core/core}%

% Dummy values

\begin{document}
\begin{bookcover}
\begin{bookcoverelement}{color}{bg whole}{orange}
\end{bookcoverelement}

\begin{bookcoverelement}{normal}{front}[,,,0.4\partheight]
\centering\bfseries\huge \pageref{combat}, \nameref{combat}\par
\end{bookcoverelement}
\end{bookcover}

\end{document}