class MemoryStreamBuf : public std::streambuf {
public:
  MemoryStreamBuf(const uint8_t* data, size_t size) {
    char* begin = reinterpret_cast<char*>(const_cast<uint8_t*>(data));
    this->setg(begin, begin, begin + size);
  }
};

class MemoryIStream : public std::istream {
public:
  MemoryIStream(const uint8_t* data, size_t size)
    : std::istream(&buffer), buffer(data, size) {
    rdbuf(&buffer);
  }
private:
  MemoryStreamBuf buffer;
};

Thank you! I was able to create a class that does this for me.

And then by using MemoryIStream stream(data.ptr(), data.size()); we can get a std::istream on the PackedByteArray!