Is anyone actually planning to do this? I'd like to know if Data Saver is enabled and there's a synthetic trial for that. I upped the priority because we need to separate trial groups (Data Saver on vs off) to evaluate some work we're doing.

There are no immediate plans to work on this, but what needs to be done is:

move the SyntheticTrialRegistry from here:
https://cs.chromium.org/chromium/src/components/metrics/metrics_service.h?rcl=4eeae7ba49f935ac91e2eca183c34bc5095540cf&l=381

to somewhere that it can outlive UkmService, and then pass it in to the UKM FieldTrialsProvider here:
https://cs.chromium.org/chromium/src/chrome/browser/metrics/chrome_metrics_service_client.cc?rcl=7a07bc981207c6a392766b782dc438419940765b&l=690

Re #2, we added a custom |save_data_enabled| boolean to UKM. Issue  827608 tracked that work. 
https://colab.corp.google.com/drive/1aybfrogFbNG4MTVYLVV3cj3mpo5lfSJ_ has an example of how to use that boolean in analysis.