This is an old comment, but this thought occurred to me as well.
I haven't decided yet if I will do it or not, however, what I would probably do is run it on both pro micro and pi. I would use a toggle switch to physically switch between the female keyboard output, and to power the raspberry pi.
However, if I needed to run both the pi and the preserve the USB keyboard functionality simultaneously, I would STILL need a toggle switch, to disconnect the keyboard from the pi so the strokes don't get sent to both OS's at the same time, even if I had the ability to do so.
Just a bit of reality I came to knocking this idea around in my head as well.
The pi is garbage for doing USB as it non-native. If you want no pro-micro and ONLY a pi, you will need to directly wire the matrix, and do it yourself. I think that amount of work isn't worth it.