You have a forward pass and you're not throwing a flag, which means you have declared it "legal". That makes it a legal forward pass. If it was illegal, you'd have a flag. You can't have a forward pass that's in the no-man's-land between legal and illegal.
2-32-ART. 11 . . .A passer is a player who throws a legal forward pass. He continues to be a passer until the legal forward pass ends or until he moves to participate in the play.
9-4-ART. 4 . . .Roughing the passer. Defensive players must make a definite effort to avoid charging into a passer, who has thrown the ball from in or behind the neutral zone, after it is clear the ball has been thrown. No defensive player shall commit any illegal personal contact foul listed in 9-4-3 against the passer.
However, let's pick a different nit. The passer ceases being a passer when then pass ends, which means *by rule* you can't have RTP after the spike hits the ground which should be "immediate". Therefore, the defensive player would have to be
Superman-ing over the offensive line at the snap trying to hit the QB before the spike was complete, and chances are that will be a personal foul anyway (if not just encroaching).
To that end, I'd say it'd be rather impractical to have an RTP on a spike play because of the necessary timing involved. However, if you call it a DBPF, the spike play counts, but calling it RTP lets the offense replay the down.
You could also one-up the penalties if the DB goes Superman -- the encroachment means the down ended before it began, and you can hit him with the DBPF on top of that. However, the clock status would remain and the clock starts on the ready (assuming the clock was running, because why else would you be spiking it?) -- but chances are you're also under 2 minutes in the half and A would have the option to start the clock on the snap instead.