I think the goal of being "sustainable" kind of rules out the CornishX for meat birds. A heavier breed (like those Delawares) that will make lots of eggs but can still get people hooked on the flavor of the "free cockerels" sound like the best option. Use the culled laying breed to give people a taste for range-fed chicken and then direct them to your meat breed for a larger carcass that they can purchase.
Depending on the market, that can work... especially with niche markets for sustainable and pastured animals, but it needs to be very carefully handled from the business standpoint, as if you approach the average uninformed consumer, or just throw it into mixed CSA box you could accidentally put off potential customers... for instance we rarely include brisket, skirt steak, blade steak, lamb shanks, lamb necks, roundsteaks, tongue, oxtails, or hearts in our $100-$200 beef/lamb sales.
My husband and I, in discussing our chickens (layer/meat/dual purpose breeds), beef, and sheep, abbatoir, market, and farm finances overall tonight came up with an excellent analogy for the dual purpose chickens.
Motorcycles... you have your road bikes, and your dirt bikes.... then you have your dual purpose like the Yamaha 500 xt... it’s not the best efficiency road bike, it is not your best bike for motocross/off road options, but it can do both adequately... don’t expect exceptional results in either area, but go for a more balanced approach.