Geany keybindings are global, they almost all work in all panes (but some fudges for Terminal IIRC). So context dependent keybindings like toggle focus don't really fit with it, especially when its related to a pane added by a plugin thats not always present.
I'm willing to add toggle shortcuts directly to the preview plugin
So yes adding the "toggle editor/preview focus" keybinding to the preview plugin that needs it is a better idea.
Geany and Geany-plugins are currently in string freeze, and so its unlikely anything will be added before release, since I expect adding a keybinding in the plugin will add a string to its UI.
Just out of interest, why do you need to shift focus to an output only preview, doesn't it update automagically?