Starting from RAW files, is the conversion to greyscale in the RAW developer then not the best option? The hues can be addressed to the grey values as well there. When you export AdobeRGB from there the 2.2 gamma is applied just once.
For myself no - the conversion choices are then limited to what is offered in the RAW converter and I can't use different conversions in different parts of the image.
It is however a distinction relevant to a small proportion of images (as is the prophoto choice). Gamma changes are not a concern for me at 16 bit (YMMV)
The only two RAW converters I use much are ACR in CS6 and DxO Optics Pro (with its colour space 'limits') so there may be others with different B&W options, but I've not tried them