Thanks majenko. That will certainly come in handy. For now though, I've decided to make the design modular. The 5V SPI device will be an add-on with its own controller, which then communicates via I2C to the main controller. This'll allow me to focus on getting the initial stage of the project done, and add other stuff later.
So this just left me with a single issue still needing to be addressed, which is converting the 3.3V signal from the uC to the 5V signal needed for the LED drivers. Two signal lines ... I may just use the schematic for SFE's level converter and build it into my circuit. I should work ... I think.