I'm not sure how you were trying to do it. But I downloaded Girdir and gave it a try.
If you set the hotkeys in XMPlay to something like:
ctrl + shift + alt + num0 = next track
You can get girder to make XMPlay think that combo has been pressed by:
Create a new command,
Under "Keyboard" put in whatever hotkey combo XMPlay uses.
Is that what you were trying to achieve? or gives you ideas?
Not sure what you meant by attaching messages... if you want to elaborate on what your trying to get Girder to do I could look into it some more.
I can't really test it properly though, don't have the right gadgets