the various LAB spaces are supposed to be perceptually linear which is computationally suprisingly nonlinear (thats also the reasons why there are so many different models, which basically try to be more linear than the others).
Writing a node for converting color values to e.g. CIE 1976 Lab* would be completely possible and cool and would make an excellent learning exercise.
But you were actually asking for a color transformation node - color transformations are internally using a 4x4 matrix multiplication. this matrix multiplication is able to do a suprisingly useful number of basic color manipulation things - but you can not express a conversion from RGB to a lab space with it. Our brain is way to nonlinear for this.