It's actually quite easy, if you know how.
Googled this up:
http://www.kerwal.com/home/installzentyal203fromausbthumbdrive , thanks, Jeremy Walker!
The easiest way is:
1. Use UNetBootin to make a bootable USB Drive (standard ubuntu utility has the "vesamenu" bug)
2. Copy the Zentyal ISO to the same USB Drive, rename it to something short (like "zen.iso") for less typing later.
3. Load from USB - standard installation.
4. When it asks to look for CD-ROM drivers on removable devices, choose "yes" (somehow it won't mount without this step)
5. After it fails, start CD-ROM lookup again, this time agree to point it manually.
6.
Alt-F2 to another terminal
7.
ls /dev and find your USB drive (if you have one hdd, the usb drive will usually be "
sdb" with one partition "
sdb1")
8.
mkdir /mnt/somename (create a directory to mount)
9.
mount /dev/sdb1 /mnt/somename (in this example USB Drive is sdb1)
10.
ls /mnt/somename (make sure it shows the contents of your USB Drive)
11.
Alt-F1 to your installation, when prompted to enter the CD-ROM device path, instead of
/dev/cdrom put
/mnt/somename/zen.iso12. When asked to unmount the USB drive, choosing "yes" won't crash the installation.
The rest should be as usual.
Just installed it from USB, it works. Feel free to re-write to make this guide more comprehensible.