The DLL is in the ROM package for the WM8505, and I copied it and the registry key HKLM/Drivers/USB/FunctionDrivers onto the ZT-180, but still no dice.
I wonder if it also needs a USB driver installed on the PC end (wceusbsh.sys), like with my TinyBook (copied from that device itself) and V212 (which C:Amie wrote for me).