You should be able to further decrease the probability of a loot table dropping by adding a dupliate mob to the spawn group without giving them the 'rare' loot table.
e.g. say you want to add a really rare drop from orcs
Spawngroup 1;
99% chance of orc with 50% chance of normal loot table
1% chance of orc with 50% chance of normal loot table, and a 1% chance of super rare loot table
Keep the mobs identical except for the loot table and it's invisible to the player. They just see an orc.
This would give you a 1/10,000 chance of the super rare loot table dropping. If you really want to go crazy, you could further decrease the probability of a single item dropping by making the super rare loot table virtually the same as the normal loot table, but add your rare as a 1% item. You could add an item with a 1 in a million chance of dropping this way.
Untested
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