The "Play All" rule is on the table. Someone plays the new rule, "Get on with it" which states that "At any time before your final play, you may discard your entire hand and draw a new hand of 3 cards. This is a free action but if you take it, your turn ends immediately." So, if this player decides to discard his remaining hand & draw 3 cards, does he still have to play them, or does his turn end immediately with new cards still in hand?
The most logical way of doing it would be to end the turn with 3 cards in hand. This is because "Get on with it" always requires you to have at least one more play, and it still causes your turn to end. There is no reason for a Play All to effect the card any differently than any other Play X rule.
Play All is like having a rule that says "Play N where N is more cards than you could possibly play", so if you use "Get On With It" it's just like interrupting any other Play N rule. Or to look at it another way, when "Get On With It" ends your turn, that's it and your turn is over so you can't play any more cards.
The short answer is the players turn ends and they will have 3 cards in hand. To simplify it don't think of it as play all but play X where X is the current number of cards in your hand. Here is a quick example that might help.
A player starts his turn with 3 cards in his hand, play 3 cards, draw 1 card and get it on with rules are in effect. The player will start by drawing 1 card bringing him 4 cards in hand and having 3 plays to make. After looking at the cards he decides that if he played any of the cards in his hand it would be bad for him and good for his opponents so he decides to use get on with it. The first part of the card has him discard his hand and draw a new hand. Next it says that this is a free action but if you take it your turn ends immediately. That would be a new rule which takes precedence over the existing play 3 cards rule.
Now you can repeat that setup and replace the play 3 cards with a play all cards and after the player has discarded and redrawn 3 cards they would still have the same number of plays left as in the play 3 setup however the get on with it card will still trump and end the turn.
As a side note this is how it works in the iPad version of the game.