Yes you are right, you cannot rig or even animate USD objects in Maya. Also as far as I know there is no easy way to convert USD objects to native Maya objects. The objects, which will be animated need to be imported as Maya geometry unfortunately.
I went through an example USD workflow, which also brings Maya animation into Houdini:
https://vimeo.com/551545616#t=1070s [
vimeo.com]
The way it works in this example:
- rigging imports the asset geo as native Maya geometry so it can be rigged
- the rigged asset gets published as .mb
- animation loads the scene.usd as USD data, which contains the shot camera, the set and character assets for reference
- animation also brings in the rigs from the .mb file and animates them
- the animated geometry gets published as anim.usdc and added as a sublayer in the scene.usd, which can be loaded by downstream departments
So the Maya stages are used in animation only for bringing in static assets and the camera for reference and playblasts. The Maya USD workflow is much more limited than in Houdini for sure, but at least downstream departments can load the scene.usd and have everything they need from the upstream departments.