Currently UNI-T multimeters (and various rebadged ones) are handled via the uni-t-dmm driver (e.g. "--driver voltcraft-vc820:conn=1a86.e008") if the UT-D04 USB/HID cable is used, and by serial-dmm (e.g. "--driver voltcraft-vc820-ser:conn=/dev/ttyUSB0") if the RS232 UT-D02 cable is used. From a user's perspective it would be nicer to have only one user-visible drivername (e.g. "voltcraft-vc820") which uses the HID backend code if a USB VID/PID is supplied and the RS232 backend code if a serial port is provided.
Revisiting this, we're planning to move the USB/HID serial emulation code into the serial.c wrappers and then merge the serial-dmm and uni-t-dmm drivers, eliminating quite some code duplication and the useless "-ser" drivername prefix at the same time. This will also help with other devices e.g. the UNI-T UT612 which we'll be able to add to the generic es51919 driver more easily.
Status update: The serial-over-HID approach indeed does work, and disguises HID cables as regular COM ports. I've successfully used UT612 with an unmodified serial-lcr driver, used the uni-t-ut61c-ser driver with a USB cable, and used UT325 with a modified driver which used "a COM port". Another byproduct was that I could use the "hid" spec as the conn= default in the driver, and have the meters auto-detected without user provided specs beyond the -d type.
Another status update: Serial over HID (and Bluetooth) support was integrated with mainline in the meantime, transparent operation of UT-D04 and other cables (Victor, Brymen) were verified. Merging uni-t-dmm into serial-dmm is possible, and will be done.