I had an LH issued award ticket and a paid ticket book by Amex travel this year. In both cases they were able to book TK seats for me (it was Business Class, I+I and J+C, it might be different for economy).

I didn't try to change anything for the LH one on the TK website. For the Amex booking, I couldn't access it from the website, but as other posters mentioned above was able to open it in the Android App (might be a new, it was 2 weeks ago). I needed the ticket number, PNR alone was not enough. With the ticket #, I was able to change seats (already preselected by Amex) and special meals. Also I was shocked to see the old date in the App after I rebooked the last segment - but after entering the ticket #, the correct new date was displayed.