> Both problems can be worked around by having a third, general-purpose host where both source/destination hosts can scp to/from.

Yup it's what I do, that 3rd computer having a fixed IP. Conveniently that computer can also keep a copy of the file(s).

Linux/BSDs/OS X (which is kinda a Unx too) all come stock with scp* and I don't really use Windows, so I'm a happy camper.

I think you could use an ssh tunnel between the intermediary and the destination such that the scp connection from the source makes it all the way through in one go, rather than leaving files on the intermediary. You'd be forwarding to the ssh port via ssh, so it would be a confusing bit of sshception.

If I tried to actually come up with the actual commands for this, I'm sure I'd burn a whole afternoon on fiddling with it.

This either requires the destination to accept inbound connections, or you'd need a permanent SSH tunnel, both of which you'd probably want to avoid.