connect Timeline AnimationPlayable output to an AnimationScriptPlayable

Yes, it's a bit complicated, and depends how you'd like to use it. If you want the animation script playable to be run as part of the timeline, you can create your own AnimationTrack and Clips which generate exactly the playable graph you need. The drawback is several of the root motion options of the regular timeline track would not be available.

Or, if you want the output of the timeline animation track to feed into a custom animation script playable, you could add it into the playableGraph generated by timeline, after the graph is created (playableDirector.RebuildGraph() will built the playable graph, and playableDirector.playableGraph is the resulting graph). This is possible, but can be tricky to get right, since you need to update both the correct branch of the graph, and the corresponding PlayableOutput.