I noticed your November post after I just posted a similar enquiry on the Multihulls forum today. I have been looking for a 34 to 38 ft max 16ft beam low displacement motor
sailer cat for a year with no real luck and wonder if it can be done. I found a couple of claims to it like http://catamarans.tartanyachts.com/models/tartan-sc-44-motorsailer-catamaran/
but noone seems to have built one yet.
The only one I know of is a NuStar but three times the displacement
I want. The Stealth cat from Thailand
may have somthing in their standard designs but showed no interest in building something similar for me.
Compucraft in Brisbane Australia
have a power cat at the yop of their web page which might handle a rag and stick and I will look further into the Peter Brady yachts you mentioned.
Let me know if you have found anything or we can share research