Both \ref and \pageref are not fully expandable, but refcount package provides a \getrefnumber{<label name>} which fully expands to the linked counter value of that <label name>. Hence your requirement can be implemented by:
\documentclass{article}\usepackage{minted}\usepackage{refcount}\makeatletter% expand value of key "highlightlines"\define@key{FV}{highlightlines}{\edef\FV@HighlightLinesList{#1}}\makeatother\begin{document}\begin{minted}[ linenos=true, escapeinside=!!, highlightlines={\getrefnumber{myline}, \getrefnumber{yourline}} ]{c++} i = i + 1 ; j = j + 1 ; !\label{myline}! k = k + 1 ; l = l + 1 ; !\label{yourline}!\end{minted}The important lines is line \ref{myline} and line \ref{yourline}.\end{document}