You mean put the reticle/crosshair abovea havrest node (only the name of the harvest node shown, nothing looted or harvested yet) and then you want to know how many you got of thsoe already in your bags?
This would someone to create manual lookup tables in all kind of languages between ahrvest node string and possible outcome materials after after harvest. I doubt there is any API that maps that already to find your matching mats in bags.
And I really doubt anyone wants to take on that manual task and keep it updated after patches etc
I might be wrong but because of this manual effort and the special updae effort I do not expect this to exist, or exsit soon, as an addon.
I bet the stuff like Loot addons that show you the info after harvesting and having the lootwindow open where the materials you will harvest are shown are the best to go with.