There are at least two ways of achieving the same result. One is to use the External Insert in the fx bin of a track or bus. The problems that I was having were due to quirky behaviour of Sonar and Audient's mixer software which is not very intuitive (not to me anyway). Sonar seems to require the solo button to be on. And it doesn't seem to work if there are any Aux track anywhere in the routing after the insert. With Aux tracks involved, put the insert on a bus instead.
The Insert's output needs to be set to Alt Out. The Alt faders need to be turned down on the mixer, but up in Sonar.
The same result can be achieved without the External Insert. Just send or output to Alt Out. Obviously the outboard gear needs to be outed into the interface inputs and a track needs to be armed for recording in Sonar.
Anyway it works, although I'm away from my DAW so I can't be specific about every detail. I've marked this thread as "solved".