Yeah but that was my point, you at the very least need a RAMpack, and often one with modifications. It is not just for having the space to have the custom display code, you actually need the RAMpack for the data lines on the bus. Without a RAMpack, the ULA would fetch the character rows from ROM and ROM only, no way around it. And not all RAMpacks worked for this, a lot (most?) had to be further modified in hardware.

The original device without RAMpack did not let you set individual pixels at all (and even with a suitable/modified RAMpack it was a giant hack obviously).