Do you know any iTunes controller application which uses its own dock icon as a GUI widget to include play/pause, next, previous buttons? Or is it possible to build one easily? If dock icons does not allow placing buttons (I've seen some advanced usages of dock icons like a progress bar or near-real time preview of virtual OS on parallels etc.), we may try to just design a dock icon containing a UI image skin but trying to receive mouse click events by other means (coordinate estimation, etc). I'd also prefer that to have Last.fm support (love/ban the song), rating functionality on dock menu.
I am not yet aware of anything like I've mentioned in the question, but HyperDock Mac application seems to be the closest thing to what I want. It adds window previews -similar to those on Win 7- to dock apps and enables special functionalities for apps like iTunes, Spotify to play/pause, next etc. You can also assign shortcuts.