configuration files that are written in JSON have the following limitations:
1 - they do not support comments
2 - keys require quotes
3 - they do not support expressions
4 - they do not allow dynamic generation of keys (for whatever reason)

As a result, in systems I was involved with there is a config folder in deed, but the files there are simple JS files that are loaded using the simple good old require mechanism.

So, for an over simplified example, instead of

{ "env" : "BR1"
, "domain" : "br1.mysite.com"
, "session": 
   { "cookie" : "br1sessid"
   , "expire" : 3000
   }
}

we write:

var ENV = process.args.NODE_MY_ENV || "dev"
module.exports = 
{ "env" : ENV.toUpperCase()
, "domain" : ENV + ".mysite.com" //on production ENV will be www
, "session": 
   { "cookie" : ENV + "sessid"  //TODO - yada yada bla bla
   , "expire" : 3000
   }
}

would such a store engine be welcomed here?
I mean, should I even start?
In any case, this store is different then file. it loads from file, but won't save, so fooling around with eval and a sandbox as implementation of parse for file formatter does not seem right to me...

Alternatively, is there a way you wish to feature to add custom stores?

If you're interested - I'd be happy to talk to somebody