Allow developers to untrack or never track an item

Here is a usecase of what I'd like to be able to do :
You want to exclude specific items from the index (don't index them) but you don't want to use the data alteration callbacks because :
1) Your items count is never going to be accurate.
2) This is happening sometimes too late.
3) You'd like to avoid loading of unnecessary entities (For the current site I'm working on, an entity load is painful because an entity can be overriden by multiple other entities).
4) Because of the 3, this slows dramatically the indexing time.