TeX does not have a special construction for bevelled fractions and thus mfrac@bevelled was implemented with our own rules. The Open Type MATH table has two parameters SkewedFractionHorizontalGap and SkewedFractionVerticalGap that could be used but we might need to rewrite a bit the code for mfrac@bevelled. One thing to decide here is that the thickness can be made arbitrary large in MathML and so I'm not sure how the two gaps can remain constant without having the slash overlapping with the numerator and denominator. (BTW, the computation of the preferred with is not very good, we have to introduce a slashMaxWidthConstant. Perhaps this can be improved too.)