Well, I did find a precompiled package in a PPA in the end: https://launchpad.net/~samrog131/+archive
I downloaded the .deb directly and did not add the PPA to my sources so I won't pull more unsupported packages than absolutely necessary. It is relatively ancient (from August 2008), but it seems to be working fine: Now I have video thumbnails in digikam.
I think I will just live with that and bide my time until something shows up in intrepid-backports or intrepid-updates. Or until Jaunty is released; whichever is first. ;-)
Btw I did install digikam 0.10 Beta from a PPA (https://launchpad.net/~digikam-experimental/+archive) - I quite like the newer version so far. I have not noticed any problems so far, but I did not do any heavy editing just yet. (To get it I had to enable two other PPAs (just follow the links), so now I have a ton of unsupported packages in my system. Oh well, I guess there is always a price.)