Yes indeed. Looking at the datasheet is always interesting. It is interesting that they allow for 9 bits OR parity. It's almost as if the 9th bit is "parity on the fly".
Anyway, this seemed to work for me, to change the 9th bit for every other byte:
void loop()
{
Serial.print ("a");
UCSR0B = 0b10011101; // turn 9th bit on
Serial.print ("a");
UCSR0B = 0b10011100; // turn 9th bit off
}
The low-order bit of the UCSRnC register (where n is the serial port) is the 9th bit. So by setting or clearing that before you send a byte, you control the contents of the 9th bit.