Can you just digitalWrite() it high and low? When the SPI hardware is not actively driving the pin, it will probably allow you to overrule it. This approach may kick the SPI off that pin, because digitalWrite() does do a lot of checking when it takes command of a pin. If that's the case, a direct port write should work.
The standard SPI library and the hardware work best with whole bytes and they don't have any mechanism to send odd numbers of bits.