I'd like to play an animated geometry in the viewport without using the playback bar.
The use case: a sort of interactive interface/visualization inside the viewport with a python state.

I know I could use python to trigger the play button, but I'd like the animation to be separated from Houdini playback (so that I can launch 2 or more different animations at different times)

I'm not sure but you can try to use Timeblend node with evaluation time set to "By time" or "By frame" and multiply the parameter by independent time variable from python.