If you're not using the main window anyway, you could just minimize XMPlay to the tray, and then use the info window hotkey when you want - when you do, it'll be on top.
Well, this kind of special navigation isn't very user friendly. Switching between windows should work universally in every program, e.g. with ALT + TAB. Hiding windows away like the info window is today leaves me pressing the info window hotkey two times to get the window focused, which isn't exactly optimal. :p
But, if you do as BoggyB tips you to:
Define a set of WM_USER + x messages for controlling XMPlay from other applications. A possible use for them (off the top of my head) is to drive XMPlay with a gamepad by using a small VB prog to capture gamepad actions and send messages to XMPlay.
it would be possible to make plugins to XMPlay, which I then could make myself. My own homegrown info window would be just fine, if it's difficult to change the existing one to fit my needs.