Reverse Sort Product Collection in Magento

I assume you are constructing the collection like this:

$_productCollection = Mage::getModel('catalog/product')->getCollection()->...;

and then calling

usort($_productCollection, ...)

it doesn't work because $_productCollection is not an array, but you figured that out from the error message (I'm just being Captain Obvious).
You can try this instead.

$productArray = (array)$_productCollection->getIterator();
usort($productArray, ...);

Looking at the files which are relevant for this, I can identify Varien_Data_Collection which is the base class and implements IteratorAggregate, so you can loop over collections using foreach($_productCollection as $product).

It does so via the method getIterator() which mainly returns new ArrayIterator($this->_items);

In contrast, a level further up extending from Varien_Data_Collection there is the class Varien_Data_Collection_Db.

This class implements the method getData() as follows:

public function getData()
{
    if ($this->_data === null) {
        $this->_renderFilters()
             ->_renderOrders()
             ->_renderLimit();
        $this->_data = $this->_fetchAll($this->_select);
        $this->_afterLoadData();
    }
    return $this->_data;
}

It explicitly uses the _renderOrders() method which adds an order clause to the select statement. This is why getData() will use your given order, but the default iteration will not.