Does anyone know of a stylesheet that will convert MathML to RTF?

That's unlikely to exist: typesetting mathematical formulas
is complex enough to avoid using a language which is best
used for tree-to-tree transformations.
If you feel adventurous, try FOP using the MathML extension
and the RTF renderer. I'd be really surprised if you get
satisfactory results for non-trivial input, though. I'd go
for using JEuclid or something like this to produce SVG or
another vector graphics format.