I am exploring the feasibility of presenting two interacting PDF files side by side. Thus far my experiments with \usereferences have managed only to produce  a lua error:

.../ConTeXt/tex/texmf-context/tex/context/base/strc-ref.lua:1394: attempt to call field 'reference' (a nil value)

stack traceback:

.../ConTeXt/tex/texmf-context/tex/context/base/strc-ref.lua:1394: in function 'identify_outer'

.../ConTeXt/tex/texmf-context/tex/context/base/strc-ref.lua:1620: in function 'identify'

.../ConTeXt/tex/texmf-context/tex/context/base/strc-ref.lua:1645: in function 'valid'

.../ConTeXt/tex/texmf-context/tex/context/base/strc-ref.lua:1667: in function 'doifelsereference'

[\directlua]:1: in main chunk

The setup—

%testA.tex

\starttext

text text.\reference[firstpage]{}

text text.\footnote[fnote]{A note.}

\stoptext

%LinkTest.tex