I doubt Reyna et al are doing this in a vacuum - a third party intermediary (i.e. coach at Manhattan Soccer Club) would have brought up the idea of a HG contract from NYCFC and gauged Jack's interest to it; this would have to be done in order to keep his NCAA eligibility (if he wanted to stay in school). If no interest from the player in a HG, or if he's focused on Europe, then NYCFC doesn't even go to the league to petition.
So for it to have gotten to the point (EoS writing a blog entry on it), it's beyond the initial stage of due diligence, and that would imply some sort of favorable response was made by Jack. He could easily still reject the HG if MLS isn't keen to splash the bucks on him, but it'd be reasonable to assume he knows a range that's been offered in the past and would be comfortable with pushing forward. Plus MLS would be crazy not to let NYCFC claim him if that's a deal-breaker with keeping him stateside (he could say it's either NYCFC or nothing because it's either playing for his home team, or heading back to the country of his birth, England).
I won't be surprised if he doesn't turn pro, but I will be surprised if MLS doesn't allow him to be HG for NYCFC.