I think that the feedrate following the spindle speed will accomplish what the OP wants to do. It establishes the relationship he needs (I think).
It may be possible to go the other way round, but that's out of my reach at the moment as I have only just begun to look at Mach flavor scripting, and I still know next to nothing about the brains capability.
However, to my mind, although Mach does calculate the travel speed, there would be no obvious way to track that reasonably with PC computing power as it is happening in real time. A second problem would be the reaction time to change spindle speed.
It would make an interesting challenge though.