This sounds overly complicated and expensive.
But apple juice or cider. Add sugar. Ferment.
The beauty of home brewing is the experimentation process. (at least for me) It's always nice to hear stories of people trying new stuff.
Edit because I'm a dummie that can't read.
Why on earth would this not be a good idea?
When I put my reading glasses on it seems smart.
You might be able to do something similar with the canned concentrate and then you could skip the reduction. I.e. just use less water than it calls for.
Or add some concentrate to your martinellis.