Well, who do you think is speaking in Isaiah 28:11??
Fill in the blank. Who is speaking in Isaiah 28:17? ___________________
I picked a verse, one out of many that could be adduced, (many examples have already been provided by others here) that meets your criteria:
1. Paul quoting God.
2. "in support of his writings".
1. In I Corinthians 14:21 Paul quotes from Isaiah 28:11 to prove a point. That satisfies your criteria, "in support of his writings"
2. Paul substitutes "says the Lord" in his quotation, which is incoherent without the assumption that the Scripture he quoted was God speaking.
3. It necessarily follows that when he quoted Isaiah 28:11 he was quoting God.
Conclusion: Paul is quoting God when he quotes Isaiah 28:11; as he makes unmistakably clear with, "... says the Lord"
Those words constitute, to use your terminology, "direct quotes from God".
Cordially,
No. I can quote Isaiah myself, but that does not make my pronouncements any more true than they are unto themselves.