Choice observer in Dazl

Can one observe exercises in Dazl, specifically ones were a party is a choice observer?

Yup, if you’re using dazl.Network:

https://digital-asset.github.io/dazl-client/dazl.client.html#dazl.client.api.AIOPartyClient.add_ledger_exercised

The newer, lighterweight API (dazl.connect) does not have the ability to observe exercise events as it aims to be closer to the Daml TypeScript library (and therefore constrained by what is exposed by the HTTP JSON API).