You could try this for mounting and unmounting disk images. I used this bat file as the emulator exe back when I used SSF for Saturn emulation and it worked great.
@echo off
set game=%1%
set daemon="C:\Program Files (x86)\DAEMON Tools Lite\DTLite.exe"
set emu="H:\Emulation\Emulators\SSF\SSF.exe"
%daemon% -mount 0, %game%
start /wait "" %emu%
%daemon% -unmount 0
Obviously change out the path to the SSF.exe to your emulator of choice. I kept the bat file in the SSF folder where the exe was just for organization but you could probably put it anywhere you like and then in Launchbox you point your emulator to the bat file instead of the exe itself. The bat file will mount the disk, run the emulator and when you close out the emulator it will umnount the disk image. Hopefully it works as well for your FM Towns emulator as it did for SSF.