Since 2.x amarok lost capability to search single track info 'cos musicbrainz support has been removed. I liked this feature tbh, and really wanna It (or something the same) back. Now the question: a gonna implement this feature (on a musicbrainz-3.x lib), but i don't really sure if it's needed by developers? If such thing has any chances to be committed?

Only if you can also maintain that on the long run, and you will soon get aware that the API you need is not that easy to use. We simply do not integrate code that has no guarantee to be maintained.
Please have a look at the various links here:http://amarok.kde.org/wiki/Development and check the hacking guide in http://gitweb.kde.org/amarok/amarok.git/tree

But first of all I think you should write a mail to amarok-devel@kde.org and present your idea.

Ok.Almost done, still need to write tonnes of comments and clean somewhere. But for a now it works ok, fixed all insects I found.
It's only 1 dependence added - libofa, needs to generate fingerprints. Track decompressing works on xine engine, so MusicDNS service available for all formats that amarok supports. Without libofa it is only MusicBrainz service available (search by track tags and filename info).
Comments and some not critical improvements coming soon (hope I will have some time for this).

http://pastebin.com/QiUAqRHt

What I still miss is a mail of you to amarok-devel@kde.org telling about your project. That should have been really the very first thing to do, be it only to make sure you are not doing redundant work and to make sure it will indeed be integrated in the code...
Code discussion is done on the mailing list, not in the forum.