With Rasplex 0.3.1 on PMS 0.9.8.10 (with PlexPass), what is viewed does not show up on "Now Playing" on the server or through the API. Rasplex is signed in to the server with a pin and claims to be logged in when I look in settings on the RasPlex box.
The feature required a merge with a later version of PlexHT that has the ability to work with Now Playing. This is currently working in an experimental (very beta) build, so it will be available in the next release.