Tnx, It works with only a kernel and initrd now.
For your info..
the sanboot command that ipxe uses (see it as an alternative to pxelinux.0) seems like to use the iSCSI / SAN protocol.
All linux versions that booted out-of-the-box contained some version of an iSCSI Initiator like open-iscsi.
There were also rules in udev to recognize such devices. There seems no open-iscsi module for Porteus or Slax so the idea to use sanboot to boot the iso over http ended there.
The sanboot command only works for an iso that has all files necessary for the boot in the kernel and initrd or has a kernel URL (iscsi/nfs/nbd/http) command for locating the other files.
Just for info. This is no feature request!! The advantage of NBD above NFS is that:
- it is faster
- possibility for load balancing (multiple servers serving one or more clients)
- not limited to disk images or folders. You could also use it for using a swap partition or even the memory of a remote system. Very useful if the client has not enough ram and a server has.
- more and more linux versions especially the tiny one's are making the step from nfs to nbd.
(That was one of my problems. I was looking for a small linux version that didn't use or could work without one of there protocols but was just simple and fast, and could work within the existing m$ environment. I found it. It is Porteus

with great help on this forum.)
Both are simple to use and work with squashed images. Also both can be used on linux and windows as host platform. So can work in most environments. But you are right. As NFS modules already exists for porteus / slax /slackware and there isn't one for nbd, it is far more easy to use nfs. There are some other advantages of nbd then I named above but would it really matter in this case.. I think the answer is no. Aldo it would be nice as feature.

Now only fix automount of usb sticks for openbox and I am happy. But I can figure out how. I still have ideas of what I could have done wrong or is missing. I already use Porteus in the production environment and haven't received any complaints. Only people saying that it boots fast. Just what it supposed to do.