If you want the conversation option available once the "unibelt" is equipped, you wouldn't want to check it against OBJECT_INVALID and return the script as false, because then the thing will never come up. Also, the item can be possessed but not equipped, so the PC could still get your node even if they didn't have the thing equipped. This script should be more of what you are looking for:
object oUniBelt = GetObjectByTag("unibelt");
object oItem = GetItemInSlot(INVENTORY_SLOT_BELT, GetFirstPC());
if ( oItem == oUniBelt)
This compiles fine, I just don't know if it will work in-game (I've never done a script like this before). However, it should work... if it doesn't, feel free to come back and tell us what problems you're having.