Edit: Eventide support is quick and helpful. I got it all working with Sonar sending the CC to start/stop.
Apparently the issue is that Sonar cannot separate MIDI start/stop from MIDI clock (i.e. you can either send all or none) which confuses the Eventide MIDI implementation when MIDI start/stop coincides with a patch chg message (which it does if you define the patch in the MIDI track inspector) ... the workaround is to call the Eventide batch change slighty after starting transport in Sonar and then all works OK ...