I am unable to add cover art to .mka files even though it's supposed to be a supported type.
I searched the forum and saw that Florian mentioned that this was on his "wish list".
However, seeing as he said that nearly 3 years ago, I do not think it's too soon to ask if there is any progress on this issue?
Having consistent capabilities across supported file types is a huge benefit (you don't have to remember what the various limitations are for different formats). I remember how delighted I was when I read that MP3Tag had added support for .WAV files (which I had assumed was not possible due to no standard for metadata in that format). This meant that I could tag ALL of the audio formats that I was using at the time, which made things so much easier.
Hopefully we can see Matroska (which is a very good container format) reach parity with the other file types soon, as it has been requested several times already (form the search results that I have seen in the forum).