From Law 3.6
We know the replacement is allowed only when the ball is dead.
There is a variation for WR rugby 7s- running sub. which I understand well.
But is there any variation for 7s that the replacement can be allowed for the team who possesses the ball when a penalty is awarded?
The 7's law variations do not mention rolling replacements, they only cover how many replacements are allowed. If there are no variations then we revert to the main laws. Alternatively competition regulations may be in place to cover this.
RFU regulations on rolling replacements state that they cannot happen at a penalty kick, the purpose of this is to stop the defending team announcing a replacement in order to slow down the attacking team from taking a quick penalty. Other competition and Union regulations may take precedence on this.
So Bly, the answer to your question is 'maybe'.
The Rugby Ref