Thanks for your suggestions.
First suggestion -no joy
Second suggestion - Limited success.
The default file type association does add to the Edit With list, however, if you change the default to another editor then the previous default disappears from the list and the new default appears.
CO is an excellent program and it is designed primarily for advanced and pro photographers - just the people that required the better round trip workflow. Please don't take this personally, however, I find it almost incomprehensible that the program designers and engineers at PhaseOne can't or won't do some of the things to make our workflow seamless. I would be ecstatic if they would create an external editor function like LR and also improve the clone/heal and vignetting. Better yet give us a radial filter.
Why don't they survey their user base, not just the top studio and commercial pros, and find out what we really want and need.
Sorry for the mini rant, hoever, after three years of beta testing and several support requests it appears there is nobody home or at least not listening.