It would be amazing to be able to access and use the the selected frames option which is usually in the export as sprite sheet dialog. There however does not seem to be a parameter in the command to use it.
Currently working on a custom sprite sheet building script that loops though each selected cels and positions them BUT it would be amazing to see this option added to the command if possible!
If there is any information to point me to where this option is in the source I would love to explore and try to help implement :D