Mini stems were all alloy wern't they ? If you can get the bolt out and the wedge out of the bottom you might be able to knock out if its really stuck. A piece of wood might be ok and not damage anything. Be careful when you support the frame and forks so nothing gets damaged though.
I don't know if anyone else can think of a good way but if all else fails it might be best to cut the stem so you can get the forks off and then get the bit left in the steerer tube macjined out.
As suggested give it a good soaking, WD40, Diesel etc. Leave it stood upside down filled up and see if that works.