Hmmmm... Just implemented the Dosbox´s serial port support into UniPCemu, making it able to make Dosbox´s directserial connection on the TCP port.
It works the same from the perspective of the client connecting on the TCP port(it sees as Dosbox-compatible nullmodem line to be connected to on the TCP port).
The only difference is that instead being connected to an in-emulator application, it's actually connected to the machine's actual directserial port (as configured).

So in that way, at least theoretically, I should be able to connect UniPCemu running Windows NT 4.0 in debug mode to another UniPCemu instance. That instance then converts the nullmodem input/output into the physical or emulated serial port.
That serial port is in this case not a real serial port (although it can be theoretically, if connected to a physical port on the motherboard), but instead the COM0COM driver that connects said connection through a nullmodem connection into another emulated COM port, where on Windows 10 (in this case) the NT 4.0 debugger(i386kd from the CD-ROM's DEBUG\I386 folder) should be listening and talking to?

Although the i386kd application seems to refuse to connect to the emulated COM port?
Edit: Hmmm.... It looks like it's just an issue of having to select a lower (lower than 10) COM port for it to connect to?
It displays the NT 4.0 CHK version number at least, although I don't know if it does much more?

Also added an option to set the transfer speed (in BPS) on the serial port itself(together with the receive/send byte routines).
So that should be able to apply proper speeds for the serial port.