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Thanks to this hint and Sony-Ericsson’s HID tool (warning: very buggy), I was able to bang together a quick and dirty iTunes Bluetooth remote control (for the Bt-enabled Z520a): ds-iTunes.hid. Send this to your phone (Bluetooth OBEX will do) and it ought to be usable immediately. The UI is passable but reasonably blurry and ugly; this is what you get for creating it entirely in Interface Builder with Unicode dingbats for button icons, and then scaling it down to fit the phone’s display. (Like I said, Q&D.)
I haven’t tried the keystrokes on iTunes for Windows, so some of them might not translate quite right. You’ll have to try it and let me know.