From what I know the present perfect continuous has two meanings:
1. An action that started in the past which is still continuing at the moment
e.g. It's 12 o'clock. They have been sleeping for 15 hours. Should I wake them up?
2. When an action has stopped but its results are obvious.
e.g. He's exhausted because he's been working in the garden.
Look the streets are wet. It has been raining.
Past Continuous we use when we know exactly the moment when something was happening in the past.
e.g. What were you doing yesterday at noon?
Time can be established by the occurence of another action.
What were you doing when the phone rang.
In the FREEZE game time is implied. i.e. What were they doing when I said FREEZE?
I don't think that any of the uses of the present perfect continuous applies here.
Only if you consider that the positions of the players are obvioius results of what they had been doing. But is a frozen position an obvioius result of an action? I wonder.
Do you still feel that the game practises the PPC? If you do , you have found a perfect game to practise that!