When I right-click a launcher plugin on the panel to edit its properties, there is an entry and a button each for choosing an icon. Instead of that combination a GtkFileChooserButton should be used.
That's not relevant, since one can use stock icon names too (for example file-chooser). Most desktop files use icon-names, so an file chooser button is not going to work. Instead we're using an icon-chooser dialog[1] to fix this problem. From there you can select icon names and absolute icon locations. [1] http://mocha.xfce.org/documentation/api/exo/ExoIconChooserDialog.html
Btw, the icon chooser dialog will be available in Xfce 4.6 (trunk), since it adds translatable strings and an extra dependency on exo.