So are you essentially getting at Purgatory and praying for folks ‘there’ (place or state, I do not know, or care)?
I suppose first it would be important to consider my answer should be read in light of understanding that the dead person is responsible for accepting or rejecting God’s invitation.
The only way for love to work, is to not force an outcome.
At the same time though, accepting that invitation needs to be done with humility and understanding that God is the one with ALL the power. There is nothing we can do that is deserving of the gift of life here on earth or eternally in God’s house.
So it is not by our power that we accept, but by God’s grace working through us (good stuff we do on earth).
So if the below can be read in that light…
If they are ‘in’ purgatory, that is ‘after judgement’ so ‘salvation’ is a given.
Do you wash your hands before eating? Thus the purpose of purgatory for the soul.
Is it possible for someone to put the soap on your hands, turn on or off the water, assist you in washing your hands?
We are asking God to assist the person in washing their soul - praying for someone ‘in’ Purgatory.
If I’m guessing your intent correctly.