In order to have locking and screen-saving decoupled, I think there
should be two custom command lines, one for the screensaver and the
other one for the desklock command. The "Lock when the screensaver
starts" option should be deactivated (greyed out) as soon as there is a
custom screensaver command defined (which should take care of whether or
not to lock the screen when starting).
The custom desklock command should only be used when an explicit screen
lock (e.g. launching "Lock Screen" from the system menu) is requested.
This makes sense. Possibly coupled with the above suggestion from
raster. So, a dropdown/ilist for each, with a button to add a new one?