Based on quovodis' suggestions, I successfully compiled the latest version from the main repo with jernej's patches and the following change:
I have no idea if video from addons is handled any differently. My test case are video files from storage media.
Good news, it works with the tvheadend addon and deinterlaces 1080i channels. However, it looks like it is also deinterlacing 1080p channels. I haven't investigated it any further yet. I am curious about the same thing as quovodis. It would be great, if you could elaborate a bit on how the code is meant to work.
After making these modifications a new issue arose - the Tanix TX6 box overheats and shuts down after aprox. 5 minutes. I removed the lid and put a fan on it, which makes it run considerably cooler (around 65 deg. C).