Software rendering
Software rendering is only possible on CD systems.
As the sprites and fix DRAM is used by the GPU during display, updating of the tiles can only be done during vblank.
The BIOSF_UPLOAD BIOS call can be used to do a DMA or loop based copy of pre-rendered pixel data from RAM to DRAM. Of course, the data needs to be in the correct format for it to be displayed properly:
Demo
Plots pixel with varying colors on the fix layer. Source code for AS and ready to burn ISO image included.