[LordHotPocketHomicide]

So I did some looking around on this topic, and I eventually found this helpful thread. I followed Powerlord's suggestion, and I wound up with the following code:

If you're curious, IsValidMulti is a custom stock that checks if a client is valid, as well as if they meet various conditions depending on what's passed to it. In this case, I use it to make sure the possessed player is alive, and to make sure the possessor is both alive and a boss (this is meant as part of a Freak Fortress ability pack).

public Action:possessed(client) //PreThink hook, used to set the client's buttons each frame.
{
	if (IsValidMulti(client, true, true, false) && IsValidMulti(possession_Possessor[client]) && FF2_HasAbility(FF2_GetBossIndex(possession_Possessor[client]), SPOOK, POSSESSION))
	{
		new buttons = GetClientButtons(possession_Possessor[client]); //Grab the buttons of whoever is controlling this player
		SetClientButtons(client, buttons); //Set the possessed player's buttons in accordance with the above
	}
}

public void SetClientButtons(int client, int buttons) //Borrowed from entity_prop_stocks and modified
{
	if (IsValidClient(client))
	{
		char datamap[32];
		GameData gc = new GameData("core.games");
		bool exists = gc.GetKeyValue("m_nButtons", datamap, sizeof(datamap));
		delete gc;

		if (!exists)
		{
			strcopy(datamap, sizeof(datamap), "m_nButtons");
		}

		SetEntProp(client, Prop_Data, datamap, buttons);
	}
}

The code is successful in letting me control when the possessed player shoots or uses alt-fire, but it doesn't do anything else. Does anyone have any ideas?