as of xfce 4.2.1 xfce-menu only allows square icons to be used properly. I will attach a patch for 4.2.1 to enable rectangular icons to be used properly, and a screenshot demonstrating it.
Created attachment 222 patch to xfce-menu 4.2.1 to enable rectangular icons to be used for the button
Created attachment 223 Screenshot of patch functionality
Hmm, I think your patch is a little bit more complicated than it needs to be, but I have no problem with adding this feature (though to the 4.3/trunk branch only; no new features in the stable branch unless required for a bug fix). Actually, I think scaling while maintaining aspect ratio is a useful thing to be able to do; I might just put something somewhat similar in libxfcegui4.
Implemented in svn trunk.