... I would worry whether placing the reef cringles several inches aft of the luff would create tension creases in the sail since the luff would not be tensioned all along its leading edge. I'm not sure you could get the luff tight without creating stress creases.

Could the vertical pin holding the gooseneck be modified or replaced with one that has an eye on top that would accept a snap shackle? That would get the reef tack located in a nice spot. (edit: Paul's set up uses that pin in a similar manner to what I was thinking)

On an earlier boat it was awkward to get the reef cringles hooked onto the horns. I had a sailmaker place two round eyes on either side of the cringle with a piece of webbing through the cringle holding them together. That way you could secure either side of the main to the horn (or to a snap shackle) depending on which side was most convenient. It might make things easier for you no matter where you put the tack fitting. (edit: these are the 'floppy rings' Paul posted pictures of, though I only bothered to attach one side and it works fine once tensioned)

On another boat my reefing tack line came all the way up from the deck IIRC - just a short line with a snap shackle on one end and the other end secured near base of the mast. My hesitation recommending that for your application is whether that puts too much stress on the external sail track (not enough forward component in the tack line) which isn't quite as robust as a mast with a built-in track.