This is open for discussion, and out of range for Thunar 1.0/xfdesktop 4.4.0. In todays Unix/Linux desktop world, it's pretty common to implement desklets as separate tools, with separate windows, whose backgrounds are synchronized with the desktop background. This is neither nice nor very efficient. Therefore I'd suggest to provide a simple way to hook into the desktop views provided by Thunar and xfdesktop, allowing applications to place abritrary desktop applets there. This could be done as a separate library, and desklets would run in separate processes to not affect the stability of the file/desktop manager. Communication could be done through the Xserver or using D-BUS.
Please try adesklets or gdesklets... Closing as 'wontfix'...
Maybe you won't fix this one, but maybe I'll do... :-)
Well, no one did it, and so let's close this finally. This would belong to xfdesktop anyway, or maybe an implementation of thunar/desktop that still does not exist.