I've decided to do away with the complicated video controller setup that I was starting to design. Instead of building what was going to effectively be a co-processor (with its own instruction set, memory, execution hardware...), I decided that I would leave it up to the processor to write the required bytes of data to the SPI controller. I will design a more generic SPI controller that supports input as well as output and 16 slaves (maybe more). I might still keep an external memory to store the initialization sequence for the display. For now, it's just an idea, code and schematics to come later...