I got it working tonight.
I used bootsdi-options and a script from signature krokodox. I then got a error in line 619 (in section [ScsiClass]) of txtsetup.sif, regarding wrong ramdrive. I changed this line, by mounting the sdiboot-image, and replace it with:
ramdisk=ramdisk.sys
saved the image and booted from it via pxe.
(to make a iso for cd-booting I had to issue the misofs-command manually)
Beautiful!
Thank you everyone who has contributed to winbuilder and this forum.
also thanks to
http://www.911cd.net and
http://ubcd4win.com/forumAnd goodbye to Bart. You've served me well!
Also: I now understand the Matryoschka reference I've come across in this forum.
I will try to summarize my experience and post it later this week.