>
> \appendtoks \mathss \to \everymathematics
>

That appears to work.  Thanks.  Perhaps I should, instead, try to use a
sans math font such as Computer Modern Bright.  I know it's not complete
(as far as sans math is concerned), but it's probably better than forcing
all math to be sans.

\setmathfont[Computer Modern Bright]

does not work.

Troy