You cannot use a staff to call IMPALE with because it is classed as a Staff not a Great Weapon. Staffs can have pointy metal or crystal bits. As quoted above, there are no further restrictions on what part of the weapon you have to hit with to make it count. A greatsword is no better than a daneaxe.
If your physrep is of a big axe, hammer or sword, you can do IMPALE with it if you have the means to call IMPALE.
If you have a physrep of a spear, you can use it to call STRIKEDOWN as a heroic call if you have STRIKEDOWNs. Could be knocking them over, could be a physrep for tripping them up. Again, no restriction on what bit you have to hit with.
It is best not to assume that there are rules when rules are not stated. That way lies complex "folk rules" by rumour. By all means decide you are only going to do hero calls with the metallic bits, but it's not a rule.
(Not a ref, but I have read the rules extensively as I used to use heavy armour and a mage staff)