épp most próbáltam, működik:
That worked for me.
Format your USB as FAT32 in GParted
Open UNetbootin and get it as far as the stage where it brings up the USB partition to install to e.g. /dev/sdb1 - Don't install the ISO, though
Leaving UNetbootin open as is, reopen GParted
Format the USB in GParted as NTFS
If GParted doesn't automatically add the "boot" flag, add it yourself
Now, go back to UNetbootin, which you've left open in the meantime, and click OK
Doing so, UNetbootin will think you are using a FAT32 partition and will let you use NTFS format.
EDIT: at this point UNetbootin complained the USB device was unmounted. Keeping UNetbootin open I ran sudo mount /dev/sdb1 /mnt. Then I hit "OK" on the still-open UNetbootin. It wrote the USB image to the USB.