It might be interesting for certain agents (such as critters, toys, seeds,...) to have an injection mode in which the agent gets injected every time the user clicks somewhere on the screen. Until they click the injection button again, right-click or perform some other 'cancel' action. During the injection phase a little icon could appear near the hand (similar to patch mode) or the crosshairs could follow the hand around.
My hidden motive here is that this would make it very easy for me to add the Blockworld blocks to the Garden Box so people can place them much more easily.
It would also help with the strategic placing of other agents such as plants or critters.