Bit of a setback with this project. The SRAM chip finally arrived and I found out my driver does it's job just fine. However, there are a couple issues.
1) The card actually USES the memory for network communications. After loading the packet driver and initializing the NIC, writes to the normal, built-in 8kB buffer located at the D000 segment will be mirrored onto the first 8kB of the add-on SRAM chip at D800. I confirmed this in DEBUG by filling 8kB at seg D000 and then reading back the same contents from seg D800. The reverse is not true, writes to D800 don't get duplicated to the factory SRAM buffer. I need to pour over the documentation of the packet driver and chip datasheets to see if this behavior can be altered with a register or something.
2) The Last Byte exhibits the same "bad or missing CON" error when trying to load it as in installable device driver in PC-DOS 3.2. It works fine in PC-DOS 3.3, although
3) it will crash if the memory is not 0 filled first. I'll examine that issue in further detail after 1)