Controlling iTunes or another Media Player via the Lotro client is not possible. However, what you are looking for is more of a general Windows application like TS Overlay just for iTunes. Apparently there is none, but I suggest trying out iTunesKeys:
http://www.mattberube.com/software/iTunesKeys/
"iTunesKeys is a system-tray utility that lets you control iTunes
with global hotkeys or from the tray icon's menu while in the background."