Recently, a Gatchaman fan on Tumblr had posted some concept art of this scrapped character. I'm not sure how many other production sketches were done of him, but it appears that the Japanese home video releases of Gatchaman II (and some rare Tatsunoko exhibitions) have these early planning notes included as extras.
There appears to be a general sort of netiquette/concern in not publicly sharing development material in the Japanese fandom, although Jack's age was revealed to be "14". It also appears his bird style could be described as "Falcon", since the original Japanese for the codename is "Hayabusa no Jack".
For anyone curious of his look, Jack was basically designed to look like a younger Joe, right down to his uniform emulating his brother's (save for a minor color difference or two, particularly in the helmet). I don't know if there's any concept art of his streetwear, but if they'd considered keeping the numbered shirts around, it's reasonable to assume he'd have a "6" one. I'm not sure if there's any concept art for Space Joker, and if there is, it appears even harder to come by.
So keeping in all this in mind, I'm not sure if the scant information available warrants even a brief article on this guy, but the concept art and whatever notes are available could be put on some sort of page. What do you all think?