* It is also possible to export animated hair into USD curves,

* What do you expect to do with an Ornatrix groom that is saved in a USD file? With which tool?

The point of saving the whole groom is to load it by Ornatrix on the various platforms where it runs. Other tools can't work with the groom, unless they decide to implement some kind of support for it. So far the only 3rd party tools that support Ornatrix hair are renderers, and these (usually) do not need the whole groom, they only need the final hair produced at the top of the stack, with its animation of course.

Correct. As you noticed, USD doesn't know about Ornatrix and having the groom baked to plain curves is the only way to get it into your USD pipeline.
Ornatrix also supports alembic, but I believe USD will just see it as plain curves as well. 

Assuming that the Ornatrix groom gets somehow saved in the .usd file, what do you need to do with that file?

Render it?

Open it in another 3D app?

If it's about rendering, this can work only if a special plugin is developed for the renderer, that will recalculate the groom as part of the rendering process. We already do that for Arnold and V-Ray.

If it's about opening in another 3D app, it may be theoretically possible to achieve this by storing in the USD a reference to the .oxg.yaml groom file as an external asset. But that would require both the tool that exports the scene into USD and the tool that imports it from USD to know how to handle the groom.

Most likely this won't work out of the box, we'll have to implement new features.

We need a concrete small example, maybe a Maya scene with a basic furball, and detailed steps what is expected to happen with that scene - how should it be exported, what should the exported file(s) contain, and what tool should consume it, and how.

Here's an example how rendering Ornatrix with the Arnold procedural currently works, avoiding exporting the full animated hair curves to Arnold:

* In the HairShape attributes under Renderer Export / Arnold there is an option "Use Procedural". It must be enabled. Optionally a full path to the external .oxg.yaml file can be specified, otherwise a default file in TEMP is used

* Either render the scene with Arnold from within Maya, or export it to an .ASS file and render it with Arnold's command-line tool

Our procedural plugin gets loaded by Arnold and generates the hair from the groom file on the fly at each frame that is being rendered.

I suppose you need something similar, maybe for another renderer and another file format.

Actually, Arnold can also export the scene into .USD and render that, but we haven't tested if that will work with our procedural.