\definehead[BookTitle][title]
\setuphead[BookTitle][style=\tfc,
color=darkred]
\starttext
\setupnotation[footnote][interaction=start,numberconversion=set 3]
%\setupnotation[footnote][interaction=start,numberconversion=romannumerals]
\startBookTitle[
title={\em Some title\note[trans]}]
\stopBookTitle
%
\footnotetext[trans]{a footnote}
\setupnotation[footnote][numberconversion=numbers]
\resetnumber[footnote]
Loren ipsum\footnote{another footnote}
\stoptext
Now, whatever set I specify, the output is an arabic numeral. (numberconversion=romannumerals works.)
Alan