This one should be obvious enough:

In XSLT 3.0 only a source XML document can be streaming -processed.

Why not an output of a (potentially other/unknown/external/independent)
transformation?

I think you can chain two or more streaming transformations with Saxon
EE, at least from the host language like Java and s9api or even relying
on SAX APIs. Not sure whether it is also possible using fn:transform.

Please, don't tell us that we don't know how to exactly formulate this
requirement, or that it is ("extremely") difficult to implement it. Both
a possible design and implementation are delivered in the concept of a
**concurrent queue**.