I think you can have rule evidence to suggest this is not legal.
4-1-2: A free kick shall also put the ball in play: c) When a free kick is
chosen following a fair catch [etc...]
4-1-3: A snap shall, if elected, put the ball in play
when a free kick is not specifiedThe "if elected" part in 4-1-3 is bothering me a bit, but the rest of the rules suggest that the free kick must be deliberately chosen and specified, otherwise it defaults to a snap.
I'm also thinking that attempting to free kick a ball that should be put in play by a snap is a snap infraction... maybe it's a false start by the kicker? Fundamentally the snapper/holder didn't commit a foul if he never moves...
Fun one! The defensive player would also be retroactively guilty of ENC once the RFP was blown?
I'm not 100% certain, but I think you could get sufficient rule justification that you don't have retroactive encroachment -- for encroachment purposes, a substitute is only a player once he's on the correct side of the neutral zone -- therefore, if the defensive line is within the 10 yard belt at the RFP, they are still "substitutes" and therefore not encroaching.
Now, they must be on their side of the neutral zone at the snap (or free kick), so this becomes a live ball foul for illegal substitution at the kick -- but not at the RFP.
The catch that I'm not sure of -- if one (or more) of the defensive lineman was also on special teams, and therefore played the previous down... can you be an illegal substitute without ever having left the field? The rule is for an entering substitute, but you wouldn't flag encroachment for a slow moving D-lineman during a hurry-up offense...