You won't find the answer that your looking for. The truth is, by the letter of the federation rule book, if the TD was not scored before the PF occured, you have to bring it back and asses the foul from the spot of the foul.
I had this exact same play this season. The only thing that helped me is that the ball carrier had either just crossed the line for a TD or it happend a second or two after the foul. Either way, it was close enough that I was able to just call it a dead ball PF.
Around here, calling this type of foul a "live ball PF treated as a dead ball PF" is common practice that is accepted by most coaches. Its one of those local interperations that really has no basis in the rule book, yet most officials are more than comfortable with the application.
I had a thread a month or so about the merits of this type of rules enforcement.
Question for the white hats.