The following used to work but fails in the latest beta (standalone):

\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