All fd:s of the channel would then use a single window (or rather,
one in each direction) for flow control.
Right.
I take it this could be useful for both directions, although with the
client in chanrge of setting everything up.
Yes. So far it seems your thoughts and mine are tracking pretty
closely, so I'm probably not totally off in the weeds.
Hm, this actually raises a potential issue: we really should have
separate back-pressure on each fd, so that failure to consume data from
a particular fd does not wedge the whole channel.