Thanks for your comment and offer peter

Coderus' N9mirror is the one I got the newer deb files from yes, I wasn't sure if I was allowed to link it here though, so I didn't. Searching Google for the debs was the first thing I tried (and how I found Coderus' mirror) but it didn't yield any results for the file versions that freemangordon specified in his description in the first post.

Anyhow, I now have the correct debs (thanks to freemangordon), but they are not yet working. As described in my earlier post, I've done the following:
- Unpack content of deb files to /etc/modprobe.d/ and /lib/dsp/ respectively,
- Copy unzipped libgstdsp.so (obtained from first post) to /usr/lib/gstreamer-0.10/,
- Uninstall gstreamer0.10-openmax
- Reboot N900.

Result is the same as it was with the 'newer' debs: video's are not played on the device anymore. When I attempt to play a h264 file (in a .mkv container) Media player states 'Unable to play media. Media format not supported'. Same goes for self-recorded video's, or any other video.

Running latest CSSU-testing (not thumb), PK53, CPU@1000mhz, DSP@600mhz.

Perhaps some other packages are interfering? I have the Extra Decoders Support package installed. No other media players though.

If I get it working, I'll post a step-by-step guide detailing how I did it