I am trying to do exactly the same thing. Did you manage to make any headway?
I'm using an nRF52832, care of a Fanstel module soldered to a (self-designed) breakout board.
This works great when connected to my smartphone. I'm able to connect, and send serial commands to turn my onboard LEDs on and off.
With the Pi, I'm able to connect, but I'm not able to pair, and I haven't gotten serial to work either. I've attempted numerous tutorials and none of them have worked for me.
I would welcome any suggestions. I'm using an RPi 3 (non-plus) with the 2017-09-07-raspbian-stretch.img image updated after the 3/2018 (3b plus) update, although I was working on this before that update was released as well.