We're experimenting with RYB mode here:
Which is definitely not CMYK but maybe what you are interested in. This only affects the smudge setting, so... basically no; MyPaint is RGB for everything outside that patch.
The second question is more feasible. If you have a CMYK palette already you can apply that as a gamut mask to the HCY wheel inside mypaint (I describe how to do that in the post above). Or just use the palette color selector. Both options would restrict you from selecting a non-CMYK color, but would not use a CMYK model to mix colors together on the screen. . . Hope that helps!