Software rendering is only possible without additionnal hardware on CD systems.
The BIOSF_UPLOAD System ROM 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:
Does NOT work on the real NeoGeo CD, screen keeps flashing. Perhaps DMA isn't possible when the video output is enabled