In the document "MediaMosa An introduction", page 22, section 4.6, states that "media files that have been transcoded previously cannot be transcoded again". Is this still the case with 2.2 RC2?

Yes you can do more then one transcoding, but what is key here is that 'Only original media files can be transcoded'. In that case you won't have a bad situation where you have transcodings of transcodings of transcodings with bad quality as result. You can do more than one transcoding as well, as long as it is from the original file.