How can I pin offline tracks at a lower bitrate with Google Play Music? I'm using an HTC One S running Android 4.2 (actually Cyanogenmod 10.1). 

I recently uploaded my entire music library to Google Play Music, and have started using the Google Music app on my phone.

I'd like to be able to "pin" a bunch of music that I can always have on my phone (I don't have a data plan). The app allows this, but you can't select the bitrate or anything like that.

Is there way to force the app to download the music at a lower bitrate so that I can store more? I'm running out of space too quickly. 

I believe it's downloading everything at 320kbps, which is overkill for a mobile device that I use with earbuds. I'd be plenty happy with 128kbps. The app allows some changes to the bitrate when streaming music, but I can't find anything related to music that's pinned on the device. 

My phone doesn't have an SD card slot, so I can't just add more storage. 

Is there any way of doing this? Failing that, are there any third party clients that could utilize my Google Music library with more customization options for pinning 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.