Damn, I hate these stupid engine limitations >_>
Btw, I tried also in a different way. I used a code on the map like this:
* IF *
Player_1 has purchased the upgrade Upgrade_TechnologyMordorFireArrows
* THEN *
Give unit "Arsenal" (An actual Arsenal of Mordor placed out-of-map) this upgrade "Upgrade_PlantShield" (which should be the upgrade granted to it by the Influence of Sauron, thus unlocking Fire arrows for Orc Archers)
However, that doesn't work, as well. The faction set to be used is Isengard, maybe that is the reason?