Or just use right-click: Once you have dragged your selection circle to the number of seedlings you want, you release the left mouse button (how it is now). Then, when you left-click on an asteroid, it will auto-route the path, if you left-click an asteroid, it will add that to the movement queue.
Hm, reading that again i just described waypoints...
But i think this method will put less strain on your finger instead of having to hold down the mouse button and trace over asteroids.
Regards,
~HKK