sadly ,winamp hotkeys only seem to work if winamp has focus ,which i think defeats the purpose . you could try creating a hot key to the winamp. exe to steal the focus enabling the play /pause etc hotkeys to work.
to create a set of hot keys for winamp. exe start winamp, and right click winamp on the taskbar ,in the jumplist right click on winamp again and select properties, in the window that opens there is an imput field for shortcut keys,
i chose ctrl+ alt+W ,its up to you which key you choose,it will give you ctrl +alt as a default anyway.
dont know if this will solve it for you but worth a shot.