That would be a very cool feature for sure but, to my knowledge, it can't be done. Maybe someone else can set me straight?
I am a little surprised that no one's mentioned it seriously. Speedtrees are the single greatest draw to someone like me but putting trees down can be tedious at best --especially if you're planting a forest.
Your suggestion has merit in some instances, but if I'm going for a forest environment that's involved in the shot I'm trying to set up it isn't practical.
To anyone who's interested, I put an issue in the feedback tracker. This is pretty much what I came up with as an improvement to the way Plants and trees are distributed. It would be far more effective to have a paint mode much like the terrain tool, or allow you to have a placement map for the given scene. You can use a grey scale map to designate where the trees should be. This allows you to quickly distribute trees in the proper area without having to constantly click and move all over the terrain to plant one species of tree, then switch to another species and repeat the process.
Instead, the user should have a plant tab where you can click and drag the species into a queue, with a separation between grass, shrubs, trees and finally rocks/boulders. You can then set the options and hit the place button. The user could then have a complete forest planted complete with paths where there are no plants/trees/grass grow. You can paint the map in Photoshop, or use a built in brush for each plant level. You make a pass for grass, then one for trees and so on. Make it so you can tweak and repeat each section without influencing the others --this way if you don't like the way the rocks are done, you can redo it without touching the trees or grass. Allow the user to paint on geometry that has been brought in so they can distribute the trees on their own custom terrains or they can use the iclone native terrains.
Here is a link to the Feedback Tracker Issue
for those who are interested.