Ok, I am not already done with this, but I think I understand things better now that I have read these 2 How-to :
Now that I know how to customize Magento’s code, I have to understand this : what classes should I overload now ? I think Rules is the place, but where are rules conditions and actions defined ?
It seems there is some dead code in /app/code/core/Mage/Rule/Model/Rule.php : I couldn’t find where the function getStartAt() is defined…
Hope this helps, and hope some help too