Temporal effects (Echo, frame-based motion blur) in the Compositor

I would like to have a compositor node to get an effect similar to After Effect's 'Echo' (which creates a trail on moving objects, similar to motion blur but not blurred). I think allowing the compositor to access previous frames' passes (regular passes or the compositor output, for feedback loops) could lead to interesting effects being made in the Compositor!

Current workaround

For the moment, I have made a Geometry Nodes group to achieve a similar effect to Echo, but since it relies on a simulation zone to keep previous frames' data, it misses easing data between frames (which the built-in Motion Blur doesn't).