This tool will force Halo to draw shadows under scenery. This will not require any modification of the map to work, and will work with animated scenery (moving leaves, etc), as well. The shadows shouldn't consume a significant amount of resources.
Remove scenery shadow limit (will post beta release if I can manage to do this)
Increase shadow distance from (around) 1.25 WU.
Maybe increase detail of shadows?
Note: If/when I post the first beta release, it may only work with HAC2, and with Windows 7 or newer installed. If you have Open Sauce or anything similar, or if you're still using Windows XP, it is unlikely that this tool will work. Sorry.