I've heard of conversion necks to make them 22" scale, but never the other way around. I would guess that such a thing could be made, but either nobody currently does or if they do then it's a very well-kept secret. Of course you could strip the body, fill in the old bridge post holes, drill new ones further back and fit a longer scale neck, but then you'd also be drastically changing the tone of the bridge pickup and it'd be an awful lot of work when custom Jazzmaster bodies are already around.
What you want to do really is make a Jazzmaster. You could make a Jazzmaster that was was exactly like that Jag bar the chrome control plate - if that's an important enough feature to you then I'm sure you could get one made to Jazzmaster size, hack off the bottom of the JM pickguard and stick the control plate in there.