Here is the command line I use for Demul to run GDI games: (this example is for plasma sword)
demul.exe -run=dc -customrom="roms\dc.zip" -image="gdis\plasma sword\disc.gdi"
This alows demul to find the dreamcast bios (custom rom) and then run the image from any folder (my gdi images are in gdis folder)
also to make Demul portable just edit the ini file: demul.ini
change all paths like this example:
From this = C:\Emulators\Demul07a\memsaves\vms00.bin
To this = .\memsaves\vms00.bin