\definefallbackfamily [testfont] [rm] [DejaVu Serif] [range=0x25C6]
\definefontfamily     [testfont] [rm] [TeX Gyre Pagella]

\setupbodyfont[dejavu]

\starttext

a : \checkedtextchar{`a}   {{\tttf ??}}\par
◆ : \checkedtextchar{"25C6}{{\tttf ??}}\par

\switchtobodyfont[testfont]

a : \checkedtextchar{`a}   {{\tttf ??}}\par
◆ : \checkedtextchar{"25C6}{{\tttf ??}}\par

\stoptext

