I imagine it would be the responsibility of a toolkit to capture the state of an app prior to a minimize and store it as a preview. It shouldn't be an applications responsibility to generate those snapshot images.
Though hidden seems like a reasonable compromise. Maybe even "max size". I wonder, is Wayland going to implement awful separate fullscreen app context behavior like X, where fullscreen apps basically crap all over the window system, and have "fullscreen" just be a borderless window, or keep that awful mechanism? A smart DE should just suppress updates if the entire display is captured by an app in chromeless mode.