Moderator Gammon asks what is the application for 9 bit SPI and asks for data sheets.
A common application is for graphic LCDs where the 9th bit is clear for a data byte and set for a command byte. Two displays using this are:
2.2 18-bit color TFT LCD display with microSD card breakout [HX8340BN] : ID 797 : $29.95 : Adafruit Industries, Unique & fun DIY electronics and kits and 1.8 Color TFT LCD display with MicroSD Card Breakout [ST7735R] : ID 358 : $19.95 : Adafruit Industries, Unique & fun DIY electronics and kits
It would be a well received extension to the current library (documentation at http://arduino.cc/en/Reference/DueExtendedSPI) if this type of addressing could be accommodated for rather than a bitbanging method.
Several AVR libraries exist - they mostly turn off the hardware SPI, toggle the line manually if needed and clock, then turn it back on. This is very hard to do if one is using the SPI library, the functionality is not exposed.
If this cannot be accomplished, there is some code for bitbanging the SAM in a way similar to the AVR but it has not been ported to a tidy library. Tom Carpenter's gLCD libary appears to do this.