How can I pin offline tracks at a lower bitrate with Google Play Music?
This is not possible in Google Play Music. You can only change the bitrate used while streaming your music.

If you are really interested in getting your music at a lower bitrate, you could have a look at the Google Play Music directory in /data/data.
There is a directory named files. That contains all mp3-files you have pinned to your device. You COULD replace them with files with a lower bitrate, but this is much work.

Alternatively you can upload your music at two different bitrates to Google Music.