\unprotect

\unexpanded\def\strc_references_smart
  {\strc_references_start_goto
   \let\currentreferencecontent\currentreferencedefault
   \strc_references_pickup_smart_goto}

\def\strc_references_pickup_smart_goto
  {\dodoublegroupempty\strc_references_pickup_goto_smart_indeed}

\starttexdefinition strc_references_pickup_goto_smart_indeed #left#right#dummy[#label]
  \leftreferencetoks
  \iffirstargument
    {#left}
  \else
    \defaultleftreferencetoks
    \let\leftofreferencecontent\empty
  \fi
  \rightreferencetoks
  \ifsecondargument
    {#right}
  \else
    \defaultrightreferencetoks
    \let\rightofreferencecontent\empty
  \fi
  \doifreferencefoundelse{#label}
    {\goto{\referencesequence}[#label]
     \strc_references_check_visibility
     \ifreferenceisvisible\else
       \space\strc_references_smart_string
     \fi}
     {\let\currentreferencecontent\dummyreference
      \goto{\referencesequence}[#label]}
  \strc_references_stop_goto
\stoptexdefinition

\newif\ifreferenceisvisible \referenceisvisiblefalse

\starttexdefinition strc_references_check_visibility
  \def\strc_references_smart_string{\labeltext\v!atpage\currentreferencepage}
  \referenceisvisiblefalse

  \ifnum\referencepagestate=\plusone %% 1 = reference on current page
    \referenceisvisibletrue
  \else
    \ifsinglesided
      \strc_references_do_relative_else\plusone
        {\def\strc_references_smart_string{\labeltext\v!nextpage}}
        {\strc_references_do_relative\minusone
         {\def\strc_references_smart_string{\labeltext\v!previouspage}}{}}
    \else
      \ifodd\realpageno
        \strc_references_do_relative_else\minusone
          {\def\strc_references_smart_string{\labeltext\v!previouspage}
           \referenceisvisibletrue}
          {\strc_references_do_relative_else\plusone
           {\def\strc_references_smart_string{\labeltext\v!nextpage}}{}}
      \else
        \strc_references_do_relative_else\minusone
          {\def\strc_references_smart_string{\labeltext\v!previouspage}}
          {\strc_references_do_relative_else\plusone
           \referenceisvisibletrue{}}
      \fi
    \fi
  \fi
\stoptexdefinition

\starttexdefinition strc_references_do_relative_else #relativepagenumber
  \ifnum\currentreferencerealpage=\numexpr\realpageno+#relativepagenumber\relax
    \expandafter\firstoftwoarguments
  \else
    \expandafter\secondoftwoarguments
  \fi
\stoptexdefinition

\let\smartref\strc_references_smart

\startinterface all
  \setinterfacevariable {previouspage} {previouspage}
  \setinterfacevariable {nextpage}     {nextpage}
\stopinterface

\setuplabeltext
  [\s!en]
  [\v!atpage=on page\nobreakspace, %% “at page” sounds weird
   \v!previouspage=on the previous page,
   \v!nextpage=on the next page]

\protect

\useMPlibrary [dum]
\setuppagenumbering [alternative=doublesided]

\starttext

      See \smartref{figure}[fig:alpha] and \smartref{figure}[fig:gamma].
\page See \smartref{figure}[fig:alpha] and \smartref{figure}[fig:gamma].

\startplacefigure [reference=fig:alpha] \externalfigure \stopplacefigure
\startplacefigure [reference=fig:beta]  \externalfigure \stopplacefigure

\page See \smartref{figure}[fig:alpha] and \smartref{figure}[fig:gamma].

\startplacefigure [reference=fig:gamma] \externalfigure \stopplacefigure
\startplacefigure [reference=fig:delta] \externalfigure \stopplacefigure

\page See \smartref{figure}[fig:alpha] and \smartref{figure}[fig:gamma].
\page See \smartref{figure}[fig:alpha] and \smartref{figure}[fig:gamma].

\stoptext