Hmm, I'm not sure this is something that's possible.  Editing the MCS configuration files isn't something you should generally do while xfce-mcs-manager is running.  It's the MCS manager that needs to reload the file, not xfdesktop, and I don't think there's a way to do that.

For changing the backdrop, usually I'd suggest leaving the filename in the config the same, but changing the file on disk.  So if you have a directory full of backdrops, you could symlink 'current-backdrop.png' to whatever you want the backdrop to be.  Then when you want to change it, delete the link, and link 'current-backdrop.png' to something else, and run 'xfdesktop -reload'.  Of course you'd want to set the backdrop image to 'current-backdrop.png' in the settings GUI prior to doing this.

Is this what you're talking about?  It's possible I'm misunderstanding you.

that would be one possible solution i guess. it feels about has hackish as the one we came up with in the irc chat though, setting the config file to a backdrop list and then altering the list file only. ()

granted i have not actually studied the source but looking at the xfdesktop backdrop dialog box (settings/apperance-settings.c), mostly the browse dialog, it looked like two lines of code could easily force just background updates, setting image_path and calling mcs_manager_notify with backdrop channel. this could not be patched into a `xfdesktop --backdrop path` type cli command instead of a browse dialog?

No, it can't.  The settings dialog is not running in the same process as xfdesktop, and there's currently no way to "talk" to the settings dialog via the command line.

At any rate, regardless of whether or not it seems like a hacky solution (I agree, it kinda is), there *is* a workable solution to your problem.