Can this be extended to dead people whom we know for sure are in heaven? I honestly don't know.
It is certain, our Lord Jesus Christ is praying for us right now.
Hebrews 7:25 New King James Version (NKJV)
25 Therefore He is also able to save [a]to the uttermost those who come to God through Him, since He always lives to make intercession for them.
The thinking of the ancient christians is that the prayers of the saints living and "dead" (but they are still actually living in heaven praying). go before the throne of God.
Revelation 8:3-4 New King James Version (NKJV)
3 Then another angel, having a golden censer, came and stood at the altar. He was given much incense, that he should offer it with the prayers of all the saints upon the golden altar which was before the throne. 4 And the smoke of the incense, with the prayers of the saints, ascended before God from the angel’s hand.
So these prayers are effectual even those saints in heaven praying for the Church on earth, the ancient church believed that even the angels intercession and help was applicable to us, Jesus was strengthened by angels during his life on this earth.
Clement of Alexandria
"In this way is he [the true Christian] always pure for prayer. He also prays in the society of angels, as being already of angelic rank, and he is never out of their holy keeping; and though he pray alone, he has the choir of the saints standing with him [in prayer]” (Miscellanies 7:12 [A.D. 208]).
Origen
"But not the high priest [Christ] alone prays for those who pray sincerely, but also the angels . . . as also the souls of the saints who have already fallen asleep” (Prayer 11 [A.D. 233]).
Cyril of Jerusalem
"Then [during the Eucharistic prayer] we make mention also of those who have already fallen asleep: first, the patriarchs, prophets, apostles, and martyrs, that through their prayers and supplications God would receive our petition” (Catechetical Lectures 23:9 [A.D. 350]).
To me it is undeniable that the prayers of other beleivers even the ancient prayers of the Apostle Paul in some way effect and have helped us. It is a mystery and we cannot claim to know "exactly" how but we can be thankful to God that when we utter a prayer for the entire Church to be "pure and spotless" before His Coming, that makes a small difference for believers.