A little more on p. 101:


Parity is not supported by the hardware, but can be implemented in software (and stored as the ninth data bit).

In order to select 9-bit transmission, transmit bit TX9 (TXSTA<6>) should be set and the ninth bit should be written to TX9D (TXSTA<0>). The ninth bit must be written before writing the 8-bit data to the TXREG register. This is because a data write to the TXREG register can result in an immediate transfer of the data to the TSR register (if the TSR is empty). In such a case, an incorrect ninth data bit may be loaded in the TSR register.
So it seems as if you can use the 9th bit for your use, unless I missed something.