Many thanks, Radhakrishnan!For anyone interested in using this (and for whom this is not obvious) you need to put the following in the preamble or .cfg file:
\makeatletter
% thin space to html thin space
%...@ifundefined{hcode}{}{\def\thinspace{\hcode{\string&\#x2009;}}}
% thin space to html non-breaking space
\...@ifundefined{hcode}{}{\def\thinspace{\hcode{\string&\#x00A0;}}}
\makeatother
