I am working on developing a truely dual purpose chicken for my farm. (Oh I pine for the lost STD Lomona.) If you only want a few birds to butcher each year than I think your on the right track with crossing Cornish to a broiler or cornishx. If your want alot of meat birds I would suggest crossing a Dark Cornish roo with better laying breed than the broilers and cornishX. Sussex, Delware or NHR hens generally lay more eggs than broilers and the CornishX. Be aware crosses don't breed true.
I have 17 hens and 2 roos currently.
HENS(3 SLW, 6 RIR, 4 NHR, 4 Buff Orpington)
ROOS(1 SLW, 1 Delaware)
My plan is similar to yours in that at one point I plan on bringing in pure cornish genes for their meat qualities. Not real sure yet on which color dark or WLR. I really want whites but good luck finding any all white cornish.
The route I plan on taking is to first cross my Del with my biggest and best laying red hens. This results in a sex linked (Males yellowish & females mostly white)cross that should mature quickly, lay very well, and have a large frame. (My Del roo is over 9lbs) I'll select hens for mainly these traits. Then cross the sex linked hens with a cornish. If all goes well I should end up with a large, meaty bird that grow quickly, will still have good laying qualities, and hopefully no health problem due to hybrid vigor. Good Lu.ck