Send network data in the background

[Magnus Henoch]

I want to send a large amount of data to a network process
(specifically, I'm hacking file transfer for jabber.el).  I want this
to happen in the background, so the user can continue doing other
things, much like process filters do for receiving data.  I could do
this with timers, trying to send an appropriately sized chunk each N
seconds, but that would be a rather hexagonal reinvention of TCP.

Is there any way to do what I want?  If not, could such a feature be
implemented in Emacs?

Magnus