Technically, you can grab a great weapon/glaive/polearm, but that varies from kingdom to kingdom; it’s like the rules for "death from behind" in that way.
Also, grabbing a glaive in tourney is almost universally frowned upon; it’s a dick move, and you DON'T pull a move like that in tourney. Somebody (specifically a Duke) is likely to enter the tourney against you (called "Ducal perogative")to spank you (literally) very hard with their sword.