I am not sure how this is made in Maemo 5 but on my N810 there is a 'Home' menu with:
Select applets...
Applet seetings >
Set background image...
Set theme...
Calibrate screen
Help
Im guessing its kind of similar menu options on n900..?
Could we just add "Set boot splashscreen..." which opens a dialog box with the list of the current video/images in /usr/share/hildon-welcome/media . Buttons in there could be Add and Delete. You add a file to the list and the tick them on and off so you can have your own playlist of images or video file.
I am no developer at all and Im just fumbling in the dark here.. but it is now started. You developers who know the system please fill in whats do able and whats not.
I personally don't like the idea that a .deb theme changes my background and boot video. A theme is icons and panel graphics. A background is a background. And boot screen is a boot screen. "I would like to be able to have this theme, that background and those two boot screens".
Great! Should I add this to some feature request or brainstorm function? (to make suggestions and developing communications easier..). If you need any help with something graphic oriented then just ask..