I was thinking of adding an accessory to the zip line like a cable car or platform that can hold weed stems or grass planks that can then travel along the zip line 

 this will add a much needed faster transport of hucked resources.

This already exists, you can carry a stack of stem or grass to the Zip line and "drop" them onto it.  They will slide to the other end and begin to pile up there.  Don't sent to many or you'll have performance issues.