OK my update on this. Since my previous attempt broke video recording and playback, and also I had made other nasty things and lost the ability to connect to wifi APs, I decided to reflash everything to return my N900 to 'just bought' state.

So I reflashed eMMC and rootfs, and then installed root, kernel-power and its utilities, and wget.

And then followed the instructions provided and this time I got it!!

I'm now able to record in 720p.

So I'd like to mess now with bitrates, but I don't know (because I have no idea of it) where to change things to set it.

Any hint about this? Or does it require to compile things?

Thanks and kudos to freemangordon.