Can a unitarian god answer prayer?

How can we influence through prayer a god who from eternity has already decreed what should happen from the beginning to the end of time?

Andrew Murray believed it could only happen by us entering into the eternal counsel of God, and that is only possible if God is Trinitarian.

If God was only one Person, shut up within Himself, there could be no thought of nearness to Him or influence on Him. But in God there are three Persons. In God we have Father and Son, who have in the Holy Spirit their living bond of unity and fellowship. When eternal Love begat the Son, and the Father gave the Son as the Second Person a place next Himself as His Equal and His Counsellor, there was a way opened for prayer and its influence in the very inmost life of Deity itself. Just as on earth, so in heaven the whole relation between Father and Son is that of giving and taking.

("Harmony with the Being of God" in With Christ in the School of Prayer)

As Fred Sanders comments on Murray:

the fact God eternally exists as Father and Son (in the unity of the Holy Spirit) means that there is an opening, a space prepared, for the structure of asking-and-granting that is prayer

Just as the Sonship of Jesus on earth may not be separated from His Sonship in heaven, even so with His prayer on earth, it is the continuation and the counterpart of His asking in heaven. The prayer of the man Christ Jesus is the link between the eternal asking of the onlybegotten Son in the bosom of the Father and the prayer of men upon earth. Prayer has its rise and its deepest source in the very Being of God. In the bosom of Deity nothing is ever done without prayer—the asking of the Son and the giving of the Father.

Crucial for Murray was to resist the urge to think of some will of God that is antecedent to the Son and the Father, or some decision that was made behind the back of the Tirinity, in the oneness of God that is not already triune. There is no such God, so there is no such divine will. The divine will is Trinitarian and is worked out according to the asking-and-granting structure revealed in the Son:

This may help us somewhat to understand how the prayer of man, coming through the Son, can have effect upon God. The decrees of God are not decisions made by Him without reference to the Son, or His petition, or the petition to be sent up through Him. By no means. The Lord Jesus is the first-begotten, the Head and Heir of all things: all things were created through Him anduntoHim, and all things consist in Him. In the counsels of the Father, the Son, as Eepresentative of all creation, had always a voice ; in the decrees of the eternal purpose there was always room left for the liberty of the Son as Mediator and Intercessor, and so for the petitions of all who draw nigh to the Father in the Son.

(pp. 222-223, Embracing the Trinity)

Incidentally, I once heard James Torrance despair of the lack of books on prayer that were strong on the intercession of Christ. Andrew Murray's book was one of the few he thought was good.