Hi Gavin,
maybe you have solved your problem yet.
\startformula
{\null}^^{194}__{38}{\rm Sr}
or {\hbox{Xe}}^^{140}__{54}.
\stopformula
Better, ConTeXt way could be the variation on \chem:
\unexpanded\def\isotope#1#2#3{\dontleavehmode\begingroup\null\lohi[left]{#2}{#3}#1\endgroup}
\isotope{Sr}{38}{194}
or (with more logical order of parameters)
\unexpanded\def\isotope#1#2#3{\dontleavehmode\begingroup\null\lohi[left]{#1}{#2}#3\endgroup}
\isotope{38}{999}{Sr}
Or for text and math modes together:
\unexpanded\def\isotope#1#2#3{%
\ifmmode\begingroup\null^^{#1}__{#2}{\rm#3}\endgroup%
\else\dontleavehmode\begingroup\null\lohi[left]{\tfx #1}{\tfx#2}\rm#3\endgroup%
\fi}
\isotope{999}{38}{Sr}
\startformula
\isotope{999}{38}{Sr}
\stopformula
You can do it either way, whichever way suits you better.
Tomáš