Why not to create your own custom content-type
I'm using a web service(API) framework that does some magic behind the scene for different content-types requests.
I want to take advantage of that by creating responses like application/verbose+json but the actual content will still be application/json.
I've implemented it, tested it from a jquery.ajax call in chrome and it all works.
Is there a reason this is wrong or might not work in production/long run?
Internet Media Types (what you call content types) form a global space of names. If you use a "custom" name, you risk that there'll be future name collisions.
If you really think your custom name is needed, please consider registering it, see http://tools.ietf.org/html/rfc6838 (there are also vendor and personal spaces).
Custom content-type hasn't raised any issue until now in production, so i'll solve this issue by saying it's safe.