I installed this with the guidance on
http://www.faq4mobiles.de/forum/noki...tallieren.html
the english version is somehow:
http://talk.maemo.org/showthread.php?t=32935

So I did it without 'bootscreen manager'.

The only change to the FAQ above:
on my N900 it is
/opt/usr/share/hildon-welcome/media
and not
/usr/share/hildon-welcome/media/
and I had to move the original Hands-v32-h264.avi somewhere else for backup because it started playing both intro videos if both
stayed in the same place (although I edited /etc/hildon-welcome.d/default.conf accordingly).
;-)

Thanks again leetut.