I found out that it was recently added here:
The related commit I found here: http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=72868144e5862168a4f829557b7a41f686b8b12d
which also included some instructions on how to use it:
2644 Complete example to create a mosaic of the first scenes:
2647 ffmpeg -i video.avi -vf select='gt(scene\,0.4)',scale=160:120,tile -frames:v 1 preview.png
2648 @end example
Luckily I built directly from source yesterday and should been able to use this new functionality right away. Unfortunately I get this error:
Assertion ref->buf->refcount > 0 failed at libavfilter/buffer.c:147
UPDATE: Apparently, this is because the amount of tiles are to much for the mosaic which apparently has a standard size based on the tile-size, also my recordings were very "dirty" (some of them DVB-recordings). First converting them to MKV fixed the latter issue.
Please note that this is very new functionality and I do not recommend to use this in production.