\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.
The \setmathfont accepts only names of predefined math fonts (e.g. “lucida”, "modern” or “palatino"), it doesn’t create the necessary tables which are needed for math in mkiv.