In brief: the methodology used by mathematicians (postulate axioms and derive theorems from those axioms, in the long-run engaging in a process of reflective equilibrium to narrow down to the right set of axioms and theorems) can also be applied in moral philosophy (and it appears to be exactly what modern moral philosophers do). Moreover, it’s not at all clear that commonly-used mathematical axioms are less controversial than commonly-used moral axioms.
It’s not clear that your claim that “[mathematics has] commonly accepted, rigorous methodologies for determining what counts as ‘domain knowledge’ [while morality] does not” is true. See this paper for relevant counterarguments: http://www.pgrim.org/philosophersannual/34articles/clarkedoanemoral.pdf
In brief: the methodology used by mathematicians (postulate axioms and derive theorems from those axioms, in the long-run engaging in a process of reflective equilibrium to narrow down to the right set of axioms and theorems) can also be applied in moral philosophy (and it appears to be exactly what modern moral philosophers do). Moreover, it’s not at all clear that commonly-used mathematical axioms are less controversial than commonly-used moral axioms.