According to the online 1917
Catholic Encyclopedia article on
Purgatory, Section IV (Invocation of Souls):
Do the souls in purgatory pray for us? May we call upon them in our needs? There is no decision of the Church on this subject, nor have the theologians pronounced with definiteness concerning the invocation of the souls in purgatory and their intercession for the living.
As far as I can tell this is still the case today and, while there are no public prayers to the Holy Souls in Purgatory, private prayers to the Holy Souls seem to be permitted. Here’s one such prayer I found online,
here:
Prayer to Holy Souls in Purgatory
Oh holy souls in purgatory, you are the certain heirs of heaven. You are most dear to Jesus as the trophies of His Precious Blooed and to Mary, mother of mercy. Obtain for me through your intercession the grace to lead a holy life, to die a happy death, and to attain to the blessedness of eternity in heaven.
Dear suffering souls, who long to be delivered in order to praise and glorify God in heaven, by your unfailing pity help me in the needs which distress me at this time, particularly {mention your petition} so that I may obtain relief and assistance from God.
In gratitude for your intercession I offer to God in your behalf the satisfactory merits of my prayer and work, my joys and sufferings of this day.
The intercession of the Holy Souls in Purgatory on behalf of the living is not without Scriptural support. As mentioned above by JaneFrances, in the story Jesus told of the rich man and poor Lazarus (Luke 16:19-31), the deceased rich man from his fiery torment interceded for his living brothers.
The
Catechism of the Catholic Church, apparently referring to the intercession of Holy Souls in Purgatory for us, says:
Our prayer for them is capable not only of helping them, but also of making their intercession for us effective. (958)
However, it is unclear to me from this passage whether “their intercession for us” occurs while they are yet in Purgatory or after they, aided by our prayers, etc., have entered Heaven.