The mplayer patch seems to work quite well with moderate sized videos (but needs DVD movies scaled down to a half original to be watchable).
Next (after hunting down the show-stopper that meant I had to do this re-boot), I plan to install GDK in a special user's home directory and see what I can achieve with SVG and, possibly, some simple Gnome applications.
If anyone is interested in having a copy of the source code, I can provide it via e-mail, if anyone wants to request it from simon dot willcocks at t-online dot de. The code will be released as (L)GPL. The whole lot is 12k lines of code, 360K uncompressed (78K compressed), plus a 528 line patch to MPlayer-1.0pre8.