batch adding logo without reencoding the whole frame area

Hi!

I have hundreds of (720p) mp4s waiting to get a small logo watermarked at the far corner, for the first 10 secs of each file.
I really want to do it in a batch and most importantly - only reencode the area where the watermark would appear.
Also - reencode only the period where the logo is shown (not that important).
As a note, I'd really like to keep the audio track untouched in the process.

What would be the best way to do it?

thanks for the help! :)