Taco, thanks for the answer. I managed to change the whole math family
into bold, but I can't get the following example working:

\switchtobodyfont[iwona-light,ss]
$$normal {\bf bold}{\bfm bold}{\bfmath bold}$$

\bf and \bfmath result in cm fonts, \bfm has no influence, no matter
what I tried to do. I grep-ed for bfmath and boldmath and tried to
replace them, but no success at all. See the attached file for my
attempts.

Thanks,
    Mojca