"It is possible to edit hildon-welcome. A howto is available here. Examples: "Open source. Open heart. Open future.", "The Power of Open."

To insert your own video follow these instructions (from the link above thanks to VDVsx):

1. Copy the video you want to play to /usr/share/hildon-welcome/media
2. Go to /etc/hildon-welcome.d/ and edit the default.conf file or create a similar one (the videos will play by the alphabetic order of the .conf files)"

I don't have any file in /etc/hildon-welcome.d anymore after i have used the bootscreen manager =(