You can't change the fact that EXP midi CC11 is reserved in Pigments. It's becasue it's the EXP modulator.
I don't know anything about Matrixbrute. Can't you change the midi CC Matrixbrute send out?
If Matrixbrute send out midi CC11, and you can't change that, then i assume it will control the EXP modulations in Pigments, and in that case i think i would do that, if you can't change Matrixbrutes midi CC output - (meaning add the pigments modulations you wan't to be controlled by midi cc11 to Pigments EXP modulator).
If Arturia included the possibility to modulate Pigments macro controls, then the EXP modulator could be used to modulate those.