Thanks for helping out, Learner!
Pen, if you go to your src folder and open config.h in Notepad, check what var is listed after "#define KEYSTONE_PLAYER_VAR". Then, use a script to set that var to the ID of item you want as your keystone. So, if you want to use a Lava Cookie as your keystone, have a script with setvar (your var) 0x26 (Lava Cookie's Item ID).
You can also change the default keystone in config.h by editing the item ID following "#define KEYSTONE_DEFAULT". I think you will still need a script setting your var to that item ID, but I'm not certain.